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mozregression is an interactive regression range finder for Mozilla nightly and inbound builds. It uses a binary search algorithm for quickly determining a changeset range corresponding to when a problem was introduced. It is widely use by Mozilla developers and other community members to help find regressions.

To get a feel for how mozregression works, see the video on


Active contributors

These people are actively working on mozregression and good people to ask questions of (or for review):

How to setup for development


Issue / feature tracking

We track mozregression bugs and feature requests inside bugzilla, in the Testing / mozregression component.

Full Query
ID Summary Priority Status
997395 Allow installing addons from a nightly location -- NEW
1084189 Cannot use IME on Nightly builds which are launched by mozregression -- NEW
1192488 mozregression GUI installer needs admin privileges -- NEW
1197106 Add some non regression tests for mozregression -- NEW
1210046 Android builds should always behave the same way when doing A/B testing -- NEW
1217443 using the skip feature is quite confusing -- NEW
1231950 Ship (or use Desktop Fx) Android ADB with mozregression -- NEW
1232037 Output results in machine-readable format (json?) -- NEW
1232877 Option to check a date range prior to a certain build -- NEW
1233339 We may need a panel to show which build has been downloaded -- NEW
1233909 mozregression should go back further than one year for some 64-bit debug js shells -- NEW
1234029 use debug builds from archives.m.o P3 NEW
1235020 [gui] final result log output worthless -- NEW
1235127 [gui] Mozregression sometimes fails to continue bisection in mozilla-inbound builds -- UNCONFIRMED
1235260 [gui] Mozregression incorrectly sets day if I paste date string after selecting text from end to beginning -- REOPENED
1236101 [meta] Bugs that block autoBisect from using mozregression -- NEW
1237057 Have a link to the mozregression-gui.exe binary on the mozregression homepage P3 NEW
1239150 Support dynamic extend reproduce range -- UNCONFIRMED
1241358 [gui] remember "bits" choose, or take the default value ​​from the configuration file -- NEW
1241365 [gui] show ~* steps left on interface -- NEW
1241376 [gui] Progress list should scroll to the last item when it overflows -- NEW
1241380 [GUI] be able to load and display a previous run -- NEW
1241919 HTTP proxy support -- NEW
1245283 mozregression and mozregression-gui aren't using the requested profile on windows 10 -- NEW
1245587 Make Android API changes simpler for mozregression to handle -- NEW
1246086 [gui] Preselect profiles folder when you click on 'browse' -- NEW
1246142 The --pref command line parameter goes silently ignored in some cases -- NEW
1246383 [gui] Mark downloaded package on Build verdict window ('skip' action) -- NEW
1246891 [gui] Allow set days of ApproxPersistChooser in Run a single build -- NEW
1246903 Add a 'go to build...' action to other menu -- UNCONFIRMED
1247061 Reusing connections to improve performance P5 UNCONFIRMED
1248138 [gui] Allow redownload if download is interrupted or "ERROR : Unable to install" -- NEW
1248239 [GUI] allow to check the builds of the initial range, as we do in the CLI -- NEW
1248636 [gui] Options description is hard to find in Global preferences -- NEW
1248690 [gui] View the parameters in "Bisecting on" details -- UNCONFIRMED
1248767 [gui] Current build closes after 3 minutes if I restart browser (in different e10s state or with add-ons disabled/enabled) -- NEW
1249737 Mozregression lost when a build fails to start -- NEW
1256317 Error while trying to bisect fennec builds -- NEW
1256379 Automatic bug number selection chose the wrong bug from an inbound range with a backout -- NEW
1256445 mozregression does not allow to execute other mach commands -- NEW
1258166 [gui] Support save as default and clear button to custom preferences and custom addons -- UNCONFIRMED
1258688 [gui] Mozregression doesn't store values of some fields ("profile", "search for fix") when I switch back and forth between forms -- NEW
1258831 [gui] Mozregression displays crash error when I try to close single build (clone-first) using X button -- NEW
1263361 Exception in thread when hitting error ResourceNotFound 404 -- REOPENED
1270893 mozregression "To resume, run..." command goes nuts if it finds nested double/single quotes -- NEW
1275499 TypeError: unsupported operand type(s) for -: '' and 'datetime.datetime' -- NEW
1277888 mozregression --build-type fennec doesn't work -- NEW
1281790 mozregression shouldn't output bug numbers when there are too many possibilities -- NEW
1306661 doesn't recover from temporary interruptions to Internet connection -- UNCONFIRMED
1308028 out of disk space not handled well (ambiguous error, no way to recover or retry) -- UNCONFIRMED
1308417 Mozregression drops bookend integration revisions on the floor -- NEW
1311474 Mozregression fails to run with pkg_resources.DistributionNotFound: colorama -- NEW
1320322 mozregression should know about earliest Thunderbird build -- NEW
1323445 Mozregression should not identify a specific commit if there are multiple commits in the final range -- NEW
1327383 Mozregression GUI doesn't withstand disabling internet connection with background downloads enabled -- NEW
1327595 Mozregression GUI somehow can't subtract date from buildID during bisection -- NEW
1327668 [gui] If I doubleclick on "Bad" button in Mozregression - it marks BOTH builds as bad -- NEW
1327669 [gui] It's possible to accidentally start background download after quick cancellation -- NEW
1334853 mozregression --launch BUILDID used near-build instead of exact -- UNCONFIRMED
1334856 mozregression can't handle restarted firefox -- UNCONFIRMED
1339492 Mozregression fails to launch OSX builds older than 2016-07-12 P3 NEW
1350527 DownloadInterrupt should be processed -- NEW
1350530 'Retest' button on past items -- NEW
1351290 Bisecting mozilla-release with dates 2016-03-19 -> 2017-03-19 doesn't seem to work on (at least) win32 -- NEW
1362127 Add option for just getting commit range for two dates without downloading / testing builds -- NEW
1365479 [gui] List recent used downloaded builds for Run a single build -- NEW
1381319 [gui] Unreadable text depending on system theme -- UNCONFIRMED
1394911 Add option to not launch Firefox -- NEW
1394919 Add option to download binaries from final release -- NEW
1395905 Allow reusing builds which were used recently -- NEW
1398903 Provide standalone APK version of mozregression GUI and mozregression for Android -- NEW
1401503 Launcher error in mozregression GUI 0.9.13 -- NEW
1407089 Add support for reading prefs from a JS file -- NEW
1407430 Unable to find regression range between two specific date -- NEW
1408696 mozregression can't verify the revisions it itself produced -- NEW
1427371 mozregression just ran entirely the wrong build for me! -- NEW
1439887 On small initial window, input verification is skipped -- NEW
1458834 Retrying a build takes ages -- NEW
1460596 [gui] Ability to save bisection session & log -- NEW
1465145 bisecting API 11 fennec on API15 Android devices isn't working -- UNCONFIRMED
1470495 Clean up obsolete branches and add some other ones -- NEW
1477781 Incorrect regression range points to wrong bug -- NEW
1477784 "Bisection informations" should say "Bisection information" -- NEW
1478217 Bisection stuck in infinite loop -- NEW
1484531 mozregression cannot find/use OpenSSL ( -- UNCONFIRMED
1484533 Provide virtualenv for cli -- UNCONFIRMED
1485981 Mozregression for x86 devices -- NEW
1487262 mozregression only outputs the topmost commit in a push. -- NEW
1488907 Unexplained long pause before launching a Firefox instance -- NEW
1499595 add option for downloading in background -- NEW
1502956 Test changeset for regression -- NEW
1503358 mozregression-gui crash: LookupError: no codec search functions registered: can't find encoding -- NEW
1504259 Trying to use mozregression with Fennec results in "Permission denied" error -- NEW
1519138 mozregression-gui: revise the last decision -- UNCONFIRMED
1520789 Binary path is not getting escaped -- NEW
1524066 --profile-persistence={clone-first,reuse} do not work on Android -- NEW
1525283 Please make it possible to resume failed downloads -- UNCONFIRMED
1536324 Errors running mozregression on Windows P3 NEW
1544735 Firefox is crashing with builds from around the 2017-07-19 P3 NEW
1548014 mozgregression --arg doesn't work with --app=jsshell P3 NEW
1550806 "Unable to start the application" when attempting a bisection when parent.lock in profile P3 REOPENED
1558462 jsshell doesn't support testing functions P3 NEW
1568086 Provide switch to pull only nightly builds/official signed builds -- NEW
1570650 mozregression should work with esr branches -- NEW
1580625 `mach mozregression` should install mozregression into its own dedicated virtualenv P3 NEW
1604963 Attach mozregression log to related bug -- NEW
1604965 Allow mozregression to select the range based on a bug in Bugzilla -- NEW
1604967 Mozregression could set the 'Regressed By' field if a specific commit was found -- NEW
1604969 mozregression could run test cases added to bugs -- NEW
1610427 Mozregression should not look for taskcluster builds more than 1 year old P3 NEW
1615667 mozregression buries its actionable output behind pages of warning-spew, for ranges that are too old for intermediate taskcluster builds P3 NEW
1622958 Add a way to specify environment variables on Android -- NEW
1631163 Updated notification is wrong when mozregression runs from repo or has pre-releases P3 UNCONFIRMED
1631167 mozregression GUI does not sanity check user-specified good/bad builds P4 UNCONFIRMED
1635680 No option to disable blinking cursor in Mozregression GUI for Mac -- NEW
1639571 Prevent selecting date before 2004-12 P4 NEW
1647533 mozregression-gui.exe detected as Trojan:Win32/Zpevdo.B -- NEW
1648260 mozregression should provide a way to intelligently prune my downloaded-builds-cache -- NEW
1666469 mozregression fails with: OSError: [Errno 30] Read-only file system: '$HOME' on macOS -- NEW
1680160 Mozregression won't copy my profile P3 NEW
1681265 moregression gui does not work firefox-l10n ja build P3 NEW
1689850 mozregression incorrectly refuses to use profile where lock file exists -- NEW
1701318 Mozregression GUI: Fontconfig and other issues -- UNCONFIRMED
1701671 DPI problems with mozregression on Win10 -- NEW
1713539 mozregression --addon startpage causes: error: Expecting property name enclosed in double quotes P3 UNCONFIRMED
1714246 Mozregression rejects user-selected dates without warning P3 UNCONFIRMED
1714682 regression range incorrectly narrowed due to cached builds colliding P3 NEW
1718280 Release mozregression-gui binary segfaults on start P3 UNCONFIRMED
1718288 Use uic/rcc instead of pyside2-uic/rcc -- UNCONFIRMED
1720277 Improve the documentation how to pass multiple prefs -- NEW
1720580 Mozregression does not open builds from 2014 or 2015 -- UNCONFIRMED
1731648 Versions are off-by-one P4 NEW
1731835 mozregression-gui does show icons for minimize, maximize, and close buttons on Linux Ubuntu 20.04 P3 NEW
1736614 regression range incorrectly narrowed to later nightly on same day P3 UNCONFIRMED
1737729 mozregression fails to used cached build that's not available for download anymore P4 NEW
1738298 mozregression can't close Firefox on my machine | ERROR: Unable to stop the application (error: kill() got an unexpected keyword argument 'timeout') -- NEW
1746919 Investigate suspiciously low mach numbers for Linux and Windows -- NEW
1748456 Support --arg for --app gve -- UNCONFIRMED
1749215 mozregression problem on Ubuntu 21.10 P3 NEW
1758171 Release list is breaking chronological order P3 NEW
1759587 mozregression-gui is broken with Qt 5.15.3 P3 UNCONFIRMED
1760467 [mozregression] --addon has no effect for WebExtensions without in manifest.json P3 UNCONFIRMED
1763188 Make mozregression work with snap builds -- NEW
1771121 mozregression cannot launch older Beta builds P3 NEW
1771664 [mozregression] "ERROR: Unable to install None" after retry P3 UNCONFIRMED
1773343 Consider using a fallback mechanism to determine source of files -- NEW
1777554 Add an option to control whether update is disabled P3 NEW
1780012 dependabot version updates not guessing Python version correctly P3 NEW
1780781 Add an option to bisect preferences -- NEW
1781813 Investigate using alternate method to modify policies before launch P3 NEW
1784493 Raspberry Pi4: Mozregression tries to download linux64-opt and fails (linux64-aarch64-opt would be correct) P3 NEW
1785084 add helper commands to set up local dev environments -- NEW
1785095 Improve matrices in CI tests -- NEW
1786602 mozregression reports stable releases as newer than pre-releases (e.g. 4.0.19 is "newer" than 5.0.0.rc1) P3 NEW
1786883 Add `lang` parameter to usage metrics -- NEW
1787502 github-bugzilla-pr-linker being decommissioned on september 2nd, 2022 -- NEW
1791093 mozregression fails to find the push log P3 NEW
1797537 mozregression-gui.exe - system error - missing file: api-ms-win-core-path-l1-1-0.dll P3 UNCONFIRMED
1799851 mozregression GUI buttons/menus/strings are not visible in high contrast mode -- NEW
1801963 Limit date range for `mozregression -n fennec` to prevent 404s P3 NEW
1816823 Mozregression on Windows does not have version information P3 NEW
1818142 Consider using elfshaker to reduce the size of binaries downloaded and stored -- NEW
1818484 mozregression should clean up the files that Firefox puts in the update directory P3 NEW
1819026 mozregression hangs (probably due to some remote service) P3 NEW
1834765 Write a documentation for GUI tool? P3 NEW
1835168 error message "Unable to find enough data to bisect." appears after successful bisection P3 NEW
1836252 Replace `BaseClass.__init__` with `super().__init__` wherever possible -- NEW
1838631 Create manually triggered workflow to build dmg with signed/notarized app bundle -- NEW
1845801 Remove unused modules from app bundle -- NEW
1846171 Confusing and time consuming startup behaviour on macOS since version 5.7.0 -- REOPENED
1848741 Mozregression-gui is not ignoring formatting changes done by ottland leading to a broader regression range P3 NEW
1849403 Linux deployment matrix should support different install requirements for each entry P3 NEW
1849643 Upgrade actions/checkout and actions/setup-python -- NEW
1853461 Bisection wizard's window titlebar is not rendered correctly on Windows P3 NEW
1855777 Drop force push from compile requirements workflow -- NEW
1855808 Bundle macOS app with one dir executable -- NEW
1857026 Print a mozilla-central pushlog for Fenix regression windows -- NEW
1857759 Bisecting stops before the end without errors P3 NEW
1857840 mozregression permission error cert9.db file in use P3 UNCONFIRMED
1857846 Mozregression Log - selecting text is buggy P3 UNCONFIRMED
1858658 ERROR: Unable to install None (error: expected str, bytes or os.PathLike object, not NoneType) P3 NEW
1863804 On Linux, Mozregression ignores accessibility settings -- UNCONFIRMED
1868011 auto-adjust bisection point bias based on the good/bad tendency -- UNCONFIRMED
1868582 Glean testGetValue API returns `null` for all metrics when run in mozregression (because `datareporting.healthreport.uploadEnabled` is set to false) P3 NEW
1876780 mozregression -g X checks X+1 build first -- NEW
1878937 Documentation does not state the need for having adb -- UNCONFIRMED
1883340 mozregression-gui on Mac cannot capture camera or microphone -- NEW
1884938 mozregression is slow on macOS because pre-downloading doesn't pre-mount the DMG -- NEW
1885707 UnicodeEncodeError: 'ascii' codec can't encode character '\xe5' in position 27: ordinal not in range(128) P3 UNCONFIRMED
1886801 Mozregression tool crashes on macOS 13 -- NEW
1887037 mozregression seems to assume maturin is installed and in $PATH when uniffi-bindgen is installed even though that isn't necessarily the case -- UNCONFIRMED
1888605 A user.js file created by mozregression was present in my daily profile folder (and messed up a few prefs) -- NEW
1897014 mozregression GUI error: IndexError: list index out of range in "mozregui\", line 93 -- UNCONFIRMED
1899515 inform user of user namespace ubuntu 24.04 required changes P2 NEW
1902350 Virtual environment is created using the wrong Python binary on macOS (GA workflows) P3 NEW
1902856 UnicodeEncodeError: 'ascii' codec can't encode characters in position 19-26: ordinal not in range(128) -- UNCONFIRMED
1902890 mozregression-gui.exe v6.1.0 detected as malware by some security venders -- UNCONFIRMED

197 Total; 197 Open (100%); 0 Resolved (0%); 0 Verified (0%);

Submitting patches

If a bug is not open already corresponding to the bug you want to fix (or the feature you want to add), please file one. After that, please open a github pull request with your code change, and set a flag in bugzilla for it to be reviewed, as described here:

Please make sure your patch conforms to PEP8 before submitting it.