In the Firefox codebase browser code can depend on toolkit code, but toolkit code should never depend on browser code since toolkit code can be used in other applications/contexts where browser code is not necessarily available. We have a no-browser-refs-in-toolkit ESLint rule that flags attempted usage of browser code in toolkit in our JS code. We should add an analogous rule to stylelint-plugin-mozilla, especially since this has recently slipped through the cracks on a review and caused a backout in bug 1942373.

This seems to have been done (partially?) in bug 1698112.

Hey Itiel - that's a good find thanks for the heads up. That test is useful, but likely won't get run by folks for a lot of style changes which could result in backouts. I think a Stylelint rule might be nicer as it'll flag the issue at the time of commit/on Phabricator during code review.