I wrote a Firefox extension for exactly this almost 20 years ago back in their XUL iteration of extensions. Everything old is new again I guess. Brings back memories.
If I were them, frankly I'd be ashamed a bit (or a lot) of bragging about it in 2020.... but that's just me, I guess.
(Nowdays [more like many a year], a Firefox user with even heavier TreeStyleTab addiction. Now, if only Mozilla would finally get its session API in order...)