Because it is FOSS and responsible for many great contributions to apis that make the web what it is. It has history that goes way back. It has been decently transparent, certainly when compared to its closest competitors. It isn’t Google. It has a massive library of extensions. They aren’t planning to deprecate manifest v2.
Don’t get me wrong, I also like other browsers and I’m looking forward to seeing what comes from the servo reboot. But Firefox is bread and butter and there is often drummed up nonsense about it.