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.


Severities are enumerated as:

  • --: Default for new bugs

  • N/A: (not applicable): Only applies to bugs of type Task or Enhancement.

  • S1: (Catastrophic) Blocks development/testing, may impact more than 25% of users, causes data loss, potential chemspill, and no workaround available

  • S2: (Serious) Major Functionality/product severely impaired and a satisfactory workaround doesn’t exist

  • S3: (Normal) Blocks non-critical functionality and a work around exists

  • S4: (Small/Trivial) minor significance, cosmetic issues, low or no impact to users

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.

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

  • A crash may be be a S1 or S2 defect, but not all crashes are critical defects

  • High severity defects (S1 or S2) do not need to be assigned immediately as they will be reviewed by Engineering Leadership, QA, and Release Management

  • 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