Fission is Mozilla's project to implement Firefox site isolation. This introduces a major architecture change in Firefox such that a unique process is allocated for the main web page and for each unique third-party iframe embedded in it, thereby safely sandboxing them and further strengthening Firefox security.
The Fission team is standing by, ready to answer your questions in the #fission:mozilla.org room on Mozilla's Matrix server.
To file a Fission bug in Bugzilla, click here to use this Fission bug template. Or file a bug and include the word “Fission” in the bug summary. The Fission team’s bug triage will find the bug, regardless of which Bugzilla component you file it in.
- Some extensions might not work fully. But please file bugs for any of these issues!
- Some video player issues on YouTube and Twitch. (bug 1619370, bug 1620341)
- Cross-site iframes appear empty when printed.
- Cross-site iframes appear empty in screenshots.
- Documents with cross-site iframes cannot enter the BFCache.
- Session history is not tracked for cross-site iframes.
- Session restore occasionally fails to restore page state, such as page zoom, scroll offset, or form data.
- DevTools support for cross-site iframes with fission is incomplete.
- Attempting to debug extensions or workers from a non-fission window can cause crashes.
- Resource exhaustion issues with large tab counts, including:
- Excessive memory usage
- File descriptor exhaustion issues
- X11 connection exhaustion on Linux
- Picture-in-Picture doesn't work on cross-origin iframes (bug 1575868)
Fission is still in active development, and can only be enabled in Firefox Nightly.
- Enable the "fission.autostart" pref in Nightly using about:config. No need to edit any other "fission.*" prefs.
- Restart Nightly.
You can verify that Fission has been enabled by hovering over a loaded tab. If the tooltip contains a "[F]", Fission is enabled.
If you encounter an issue while using Fission, it is possible to open a non-fission window within the same browsing session using the "New Non-Fission Window" item in the hamburger menu. This can be useful to determine if issues are Fission-specific, or to work around fission-specific breakage.
To disable Fission completely, reset the "fission.autostart" pref back to "false" and restart Nightly.
|M1||OOP iframe framework||2019-02-28|
|M2||Semi-functional OOP iframe||2019-05-06|
|M3||Whistler All Hands demo||2019-06-26|
|M4||90% of mochitests are passing||2019-11-12|
|M4.1||100% of mochitests are passing||TBD|
|M5||Fission is dogfoodable (internal Fx user testing)||2020-05-29|
|M6||Enable in Nightly on desktop (X% rollout)||2020 H2|
|M7||Enable in Beta on desktop (X% rollout)||2020 Q4|
|MVP||Enable in Release on desktop (X% rollout)||2021 H1|
|-||Start work on Android Fission||2021 H1|