In Bug 1731146 we have introduced a lazy getter for the BuiltInThemes.jsm in aboutaddons.js, but while aboutaddons.js is implemented at toolkit level BuiltInThemes.jsm is actually defined at browser level.

At the time Bug 1731146 has landed, Thunderbird engineers followed up with Bug 1733451 which has introduced a Thunderbird's own BuiltInThemes.jsm module.

As a side note: Fenix doesn't use about:addons at all even if it is part of toolkit (Fenix instead uses a native android UI), and so the missing BuiltInThemes.jsm in toolkit didn't technically trigger a regression on mobile.

More recently as part of Colorway Closed set of changes to about:addons (tracked by Bug 1769138 meta) we had to duplicate some fluent strings to avoid including a desktop-only fluent file directly into aboutaddons.html (See the phabricator comment from :dao in D148431).

This follow up is meant to track work to make sure non-toolkit resources that may be needed in Firefox builds are all optional and do not break the about:addons page in a "pure" toolkit build.

The severity field is not set for this bug.
:mixedpuppy, could you have a look please?

For more information, please visit auto_nag documentation.