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+Defect Severity
+===============
+
+Definition
+----------
+
+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.
+
+Values
+------
+
+Severity levels and their definitions are enumerated at https://wiki.mozilla.org/BMO/UserGuide/BugFields#bug_severity.
+
+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 <https://bugzilla.mozilla.org/show_bug.cgi?id=1640441>`__ - Slack hangs
+   indefinitely in a onResize loop
+-  `Bug 1645651 <https://bugzilla.mozilla.org/show_bug.cgi?id=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 <https://bugzilla.mozilla.org/show_bug.cgi?id=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 <https://bugzilla.mozilla.org/show_bug.cgi?id=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 <https://bugzilla.mozilla.org/show_bug.cgi?id=1634171>`__ - Visual artifacts around circular images
+
+Bugs reported from QA
+
+-  `Bug 1635105 <https://bugzilla.mozilla.org/show_bug.cgi?id=1635105>`__ because
+   the associated steps to reproduce are uncommon,
+   and the issue is no longer reproducible after refresh.
+-  `Bug 1636063 <https://bugzilla.mozilla.org/show_bug.cgi?id=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``