I've made Firefox my default browser on Android a couple of months ago and I'd say it's basically completely usable at this point.
I am running it with NoScript on, which is probably more of an exercise in masochism than anything else - more or less every site loads broken, of course, without JS these days, and a recent change to the Firefox addon interface means selectively enabling scripts to get it working is a multi-tap pain in the ass. I am probably going to switch to uBlock Origin (which someone here mentioned works now in FF mobile).
Performance wise I'd say it's probably a bit slower but still perfectly usable (running it on a Pixel 3 and a Motorola OneVision).
I enabled sync on it as well, with an account created just to sync between my two mobile devices, and it works fine.