Desktop Firefox Reporting and Writing Good Bugs

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Thank You!

So, you think have found a bug in Firefox and you want to report it to be fixed. Great and thank you. The Mozilla projects rely on users to find and report bugs. However, you need to make sure that you write a good bug report on Bugzilla ( in order for it to be useful. These guidelines will help you write a good bug.

Note: These instructions are written for Firefox, but similar ones apply for most Thunderbird and Seamonkey bugs, only details differ. Hopefully, a Thunderbird version of this page will be released with Thunderbird 3.


Please read For All Bugs thoroughly before going to your specific issue. The information contained there is critical to making sure your issue is with Firefox, and not with an extension or corruption in your profile.

For all Bugs

No matter what type of bug you have found, make sure to follow these steps to confirm that it is a bug in a Firefox, and not an extension. To do that, follow these steps:

  1. Update Firefox, add-ons and plugins to the most recent versions (this includes programs like Flash and Acrobat Reader). All of your plugins must be on the latest supported version, as well as Firefox.
  2. Start Firefox in Safe Mode and try to find the bug. If the issue goes away:
    1. Disable your extensions/themes one by one, until you find the one causing the issue.
    2. Contact the developer of that extension.
    3. Read below for more information on Extensions.
  3. If the issue is still there in Safe Mode, make a new profile with no extensions or changes (instructions here). Then try to find the problem. If the issue goes away, your Firefox install was most likely corrupted. Migrate your bookmarks and other information to this new profile, and you should be good to go.
  4. You can read Basic Troubleshooting for more steps to try.
  5. If the issue is still present after these steps, it is likely your problem is with Firefox. However, make sure to go to and search for your issue there. Many problems can be solved with simple end-user support steps, and do not need to be reported as bugs. If you can not find any information to help on Mozilla support, keep reading to make sure your issue is really a bug, and to write a good helpful bug report.
  6. One thing to help developers is to use the latest nightly development build. These are the absolute latest in development on the Firefox program and help developers know if the issue has already been fixed. Also, if a patch is written, it will be put in the trunk first, making it easy to test. You can find trunk builds at the Mozilla FTP site.

Extensions, Themes and Other Applications

If you are experiencing a problem with an add-on, plugin, or other programs, please follow these steps:

  1. Update to the latest version. You can use Firefox's handy Add-on update manager to update extensions and themes. Some plugins and outside applications will have to be updated internally or manually from their website. It is very important that you are always using the latest version of plugins like Adobe Acrobat Reader, Flash Player, Real Player, VideoLAN and the like.
  2. Read troubleshooting extensions and themes for more information.
  3. If the problem lies with the extension (this can be proved using the steps listed for all bugs), then you should contact the developer of the extension or other application. Mozilla can not fix someone else's program for them.
  4. If the issue lies with how Firefox uses the extension, then go ahead and report it to both Mozilla and the extension developer.

Choose your Issue

After reading the information for All bugs above, choose your issue below.

  1. Crash
  2. Enhancement
  3. Internet Connectivity Issue
  4. Memory Leak
  5. Performance Issue
  6. Regression
  7. Webpage
  8. Window Shakes
  9. For Bugs Not Listed


If Firefox crashes, and you have already followed the steps for all bugs above, fill out a bug report with the following information:

  1. Try to reproduce the bug (what you did to make the crash occur). If possible, try to reproduce it on different computers with the same version of Firefox, and with the current nightly development build of Firefox.
  2. Make sure you turn on the crash reporter, and provide the crash ids with your bug report (those can be found by typing about:crashes in your URL bar and copying the appropriate number).
  3. Make sure to read how to get a stacktrace and more information that is important for developers.
  4. Now that you have all this information, create a bug with a severity of Critical. Give as much information as possible.


When you would like a feature added to Firefox, it is called an Enhancement and the severity is marked as such. To file a good enhancement request, simply follow these two steps:

  1. Make sure you have explored all possible settings and other areas of Firefox to confirm that what you want does not already exist.
  2. Give a clear description on what, where and how this should be changed. Give the dialog box to be improved, maybe even a screenshot if it is a little known feature or it would assist in understanding. Remember, there are many areas of Firefox, so don't be offended if the Triage team asks for more details or why this should be added.

Internet Connectivity Issue

If you are having issues connecting to the Internet, it is most likely not an issue with Firefox, but an incorrectly set up Firewall. Make sure your Internet settings are set up correctly, and follow the steps listed for all bugs, then search the Firefox Support site. They will let you know if it is something you should report on Bugzilla or not.

Memory Leak

Memory Leaks are very difficult to diagnose, so make sure the issue is not with your machine. Then provide the following information:

  1. Detailed information on your machine, what is happening when the leak occurs, and what seems to cause it.
  2. Read the following information found on David Barron's blog (Part 1, Part 2)and the Mozilla Developer Center on writing good memory leak bugs.

Performance Issue (High CPU use, hard drive use, etc)

Performance issues are also difficult to confirm and diagnose. The only real way to confirm them is to have a website (create a testcase, read here for more info) that we can use to confirm on any similar machine. Please try to test on at least one other machine of the same or similar configuration before reporting. Also, issues such as startup times are almost impossible to confirm and can be influenced by other factors, so please make a strong case before submitting. Developers are always working on performance, so your problem may be fixed in the next release of Firefox.


If a bug has just recently occurred in Firefox that was not there before and is caused by an update, it is a regression. These usually only happen on the Trunk builds, though occasionally they creep onto the Releases. To properly report a regression:

  1. Follow the steps listed for all bugs. Also, make sure you did not make a change recently to the settings or something else that could have caused this.
  2. Install an older version of Firefox (starting with the most recent) to find out where the bug crept in. When you find it, then file that information.

Webpage Displaying Incorrectly

Make sure that the issue you are seeing is a Firefox problem, and easy way to do this is follow these steps:

  1. Test in another browser such as Internet Explorer, Opera or Safari. If the issue is still there, it is probably something wrong with the website and they should be contacted.
  2. Follow the steps for all bugs. Extensions can mess with the rendering of a website, especially audio and video.
  3. Try to reproduce the issue on another computer running Firefox. You can also adjust your screen resolution to see if that is causing the issue.
  4. Try contacting the webmaster in charge of the website. Inform them of the problem, and ask if they know what could be causing it (some sites only work with certain browsers).
  5. Finally, if the issue is still there, make a testcase that shows in the bare minimum what the problem it. Information on how to can be found here.

Window Shakes

If you Firefox window begins to shake, vibrate, or move around without any action on your part, it is usually fixed easily by reading these webpages:

  1. Follow the steps for all bugs. Extensions can cause window shakes.
  2. Read this support document, Window Shakes.

For Bugs not Listed

If the issue you are experiencing does not fall into these specific categories, follow the steps for all bugs, then write a clear, concise bug report. Make sure you eliminate any unnecessary information that could distract from the bug itself, and give a detailed steps to reproduce with all the information you think is necessary. Remember, no bug is too little to report and it will never get fixed unless it it reported.

Using Bugzilla

Now that you have followed all these steps, found that your problem is with Firefox, and gathered the proper information, you can submit to bugzilla. When you submit a bug, make sure you read Bug Filing Guidelines. This provides basic information to make sure you fill the bug out completely Also, read the Etiquette and Privacy Policy. Before filing any bug, make sure to search thoroughly for any duplicates of the same issue. Keep your bug reports down to only one issue per bug report, it may be closed because of too many issues in one report.

You can visit the following pages for more information on bug reports and good related resources:

  1. Bug Writing Guidelines
  2. Mozilla Support
  3. Mozilla Developer Center
  4. Firefox Trunk builds
  5. Bugzilla

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Original document information

  • Author(s): Aakash Desai
  • Date last modified: May 27, 2010 at 1:45 pm PST