First thought was to try
Safe Mode, but... that may or may not be of help, because once you restart FF, extensions (then being enabled) might adversely affect what you see? Worth a shot, anyhow.
(Or you could change, or set up a separate desktop shortcut with the -safe-mode switch, in which case you would remain in Safe Mode, & that way extensions can safely be ruled out.)
Alternatively, you might disable all extensions & test that way.
(Similarly extensions would remain disabled across a restart.)
Or as a test, you could create a new, clean Profile - no changes, & see how that behaves.
Fire 750, bring back 250.
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