CloudServices/Sync/File a desktop bug

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We always appreciate it when users let us know about problems they’re experiencing with Sync because it means we can improve it! Here are some guidelines on how to file a good Sync bug:

Get the Sync log on the machine you’re having problems

  1. In the Firefox address bar, type about:sync-log and press Enter. A list of Sync logs should appear.
  2. Find the last log file entry in the list and click on it. It should be called something like error-sync-1307991874508.log.
  3. Save it to a file on disk so you can upload to the bug later.
  4. If the list in step 2 is empty or only contains outdated logs (you can tell by the dates), go to about:config, find the services.sync.log.appender.file.logOnSuccess preference, set it to true, and sync again. Then repeat these steps (although the filename will now be called something like success-sync-1307991874508.log

Get detailed Sync logs

Occasionally you may be asked to provide detailed logs (aka "Trace logs"). Note that these logs may include personal information, such as the URLs of tabs you have open or that you have bookmarked. If this level of detail concerns you, please ask in the bug that it be changed to "Mozilla Confidential" before you upload the logs - this will ensure that only Mozilla employees have access to this data.

The easiest way to get these logs is via the about-sync addon. You should:

  • Install the above addon.
  • Enter about:sync into your address bar.
  • At the top of the about:sync page, under "General Options", change the Please describe how you intend using about:sync setting to Actively looking for issues and want detailed logging.
  • If you are able to reproduce your issue at will, near the top of the about:sync page, under "Log Files and Diagnostics", select "remove all log files" - this will ensure the logs you upload have the most relevant information.
  • Grab the log files - near the top of the about:sync page, under "Log Files and Diagnostics", use "Download a combined summary as a zip file" which will download a file to your Downloads directory.
  • Attach this file to a bug - but see above - these logs may include personal information. Only upload them to bugs marked as "Mozilla Confidential" if you have concerns about information in these logs.

File a bug

  1. Even if you’ve found an existing bug that may look like the one you’re experiencing, do not add your comments to it. Please always file a new bug, we will dupe it appropriately once we’ve determined the root issue. If you want, you can clone the existing bug or make a cross-reference to it in the summary.
  2. Attach the log file(s) you saved earlier to the bug by uploading an attachment. Do not copy & paste the text into the comment field, it won’t hold enough text!

We’re hoping to automate much of this in upcoming versions of Firefox, so that a Sync problem can automatically be reported to us, or at least with just a few clicks. In the meantime, the about-sync addon offers a facility for creating a .zip file with all your Sync logs which might be helpful.

Other/Advanced information

Capturing logs from stdout

In unusual cases, such as Firefox crashing during a sync, Firefox may not manage to save the logs to a file, so it may be necessary to grab logs from stdout. This is a fairly advanced operation, so please don't feel bad if you are unable to grab these logs. The steps you should follow are:

  • Use about:config to set the following properties:
    • browser.dom.window.dump.enabled = true
    • services.sync.log.appender.dump = Trace
    • services.sync.log.logger.engine = Trace
    • services.sync.log.logger = Trace
      Note that all the above preferences should already exist - they just have different default values.
  • Quit Firefox and ensure no other Firefox instances are running.
  • Determine where Firefox is installed on your system. On Mac's, it's probably in /Applications/Firefox and on Windows it's probably in C:\Program Files\Mozilla Firefox or similar. If you are running Linux, hopefully you already know how to locate this.
  • Open a "terminal/command prompt". On a Mac there's probably a terminal icon in your dock. On Windows, you should be able to find a Command Prompt icon in your start menu.
  • Execute the following command:
    • On Windows: "c:\Program Files\Mozilla Firefox\Firefox.exe" -console 2>&1 > c:\temp\firefox-log.txt
    • On Mac: /Applications/ 2>&1 > /tmp/firefox-log.txt
      Note that in both cases you may need to adjust some paths accordingly - both the path to the binary and the path where you want the log file written.
  • Reproduce your problem, then exit Firefox.
  • The log file specified above (ie, c:\temp\firefox-log.txt or /tmp/firefox-log.txt in the above examples) should contain the log output.


This page was unashamedly stolen from - thanks to Philikon for his earlier work on Sync itself and for the initial version of this text.