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Really? I removed my tab bar years ago (I use Tree Style Tabs) and never had to tweak my userChrome.css once.

I remember it breaking exactly once. Last year they changed the syntax. But before that it was running fine for 3 or so.

Oh no! I am on ESR, so I guess I haven't gotten this change yet. How did you fix it?

Here is the change (as reflected in the sidebery wiki):

It was Firefox version 113 that changed it.