Because Firefox honestly used to be shit, especially in the early Phoenix/Firebird days, but now it isn't anymore, and they just haven't bothered to check it out again. The "killing all the existing extensions" thing really didn't help matters either.
I agree it was a heavy blow when they announced that, I didn't update for a few months, but man was it a nice fresh start, and I'm grateful that all the addons I had before have now made it over to the new system.
I admittedly didn't use FF before it was FF, I think I got into it around probably 2006 or so, but it's been great in my opinion ever since then.
I was really sour about the Mr Robot prank where they modified pages without informing the user. But weighing a prank gone wrong against the active goal of dicking over the internet I'll take the well meaning pranksters.
I have one thing missing from Firefox that keeps me from permanently switching: vertical tabs. I know there's ways of hacking it into FF, but nothing sustainable. I know it seems minor, but in Brave/Edge, it just works so damn well.
Epiphany uses WebKit, but it doesn't have working add-on support yet, so you will still have lots of ads and cookie banners.
I use it as a secondary browser if something doesn't work in Librewolf, but for me the internet is pretty much unusable without "i (still) don't care about cookies" and UBlock Origin, so it only works as a backup for me.
There are also some Browsers using QTWebengine which uses Blink (the core of Chromium (meaning technically not chromium itself)) like Qutebrowser and Falkon.
Pale Moon and Basilisk which are based on an old version of Firefox and diverged quite a bit.
Konqueror which uses KHTML (predecessor of WebKit (predecessor of Blink)).
NetSurf idk haven't tried it.
Then there is stuff like Ladybird, which will crash all the time. Links, ELinks, Lynx, w3m, Links2 are text based. Discontinued Internet Explorer. Old versions of MS Egde. Old versions of Opera.
Basically nothing else that is usable.
Concerned no I think... But I am gonna guess you missing on some features and specially extensions but I might be wrong. Except on iOs since there you are missing on that anyway.
I’m willing to sacrifice some FOSS for convenience, as long as it’s reasonable. But in general I’m a strong supporter of open source and publish my own code that way.