Defect Severity


We use the severity field in Bugzilla to indicate the scope of a bug’s effect on Firefox.

The field is display alongside the bug’s priority.


Severity levels and their definitions are enumerated at

By default, new bugs have a severity of --.

Examples of S1 bugs

  • WebExtensions disabled for all users

  • Web search not working from URL bar

  • Crashes with data loss

Examples of S2 bugs

Bugs that could reasonably be expected to cause a Firefox user to switch browsers, either because the severity is bad enough, or the frequency of occurrence is high enough.

  • Bug 1640441 - Slack hangs indefinitely in a onResize loop

  • Bug 1645651 - Changes in Reddit’s comment section JS code makes selecting text slow on Nightly

Bugs involving contractual partners (if not an S1)

Bugs reported from QA

  • Bug 1640913 - because an important message is not visible with the dark theme. It’s not marked as S1 since the issue is reproducible only on one OS and the functionality is not affected.

  • Bug 1641521 - because videos are not working on several sites with ETP on (default). This is not an S1 since turning ETP off fixes the issue.

Fuzzblocker bugs, which prevent fuzzing from making progress

Examples of S3 bugs

Bugs filed by contributors as part of daily refactoring and maintenance of the code base.

Bug 1634171 - Visual artifacts around circular images

Bugs reported from QA

  • Bug 1635105 because the associated steps to reproduce are uncommon, and the issue is no longer reproducible after refresh.

  • Bug 1636063 since it’s reproducible only on a specific web app, and only with a particular set of configurations.

Rules of thumb

  • The severity of most bugs of type task and enhancement will be N/A

  • Do not assign bugs of type defect the severity N/A