I can't speak for everyone but the fact that it appears suddenly at all is rather annoying. It's like a blast of cosmic rays aimed right at my Error-Producing memory. You can tell me that I won a billion dollars and the solution to the Kryptos puzzle and I would still seethe over forgetting the what band I was about to look up.
That said Firefox is pretty good at obeying its own "Recommend me new features" option.
If this was the only feature FF has ever shoved down my throat I would be fine with it, but without the disabling the feature recommendation option, every other time I open a tab I get hit with a pop-up for some minor UI change. The worst part is that it stops typing so sometimes I was just typing into the abyss for 5 minutes before I realize it lost everything I typed.
I've been using since 3 and I don't think it's a miracle browser or anything. Anecdotally I've observed plenty of folks try FF for bit in the wake of Manifest v3 then switch to Edge/Brave, and it's not because of a lack of AI. They need this sidebar but it's not the only reason they're losing.
I don't have the same experience as you with FF and nothing like losing what I type ever happened to me. That would indeed be nasty bug and make it very unreliable/untrustworthy. It's possible you have something corrupted? Try a reinstall or try on a new machine and see if you can replicate the bug? I'm not 100% with Firefox but it's really the least bad option to me. I found, for example, quite irritating how FF was playing shenanigans with the new tab/homepage settings, eagerly recommending things I never asked for but luckily I was able to disable that. So far FF is quiet, not obnoxious and working.
I don't like shoved down user throats features but i feel that Firefox has to evolve with times or simply wither. It's important how they do proceed with that tough. So far they're not abusive in behavior. I'm back to Firefox after giving up in Chrome and I'm quite pleased with it. Firefox is best for me.