With something as wide scoped as a browser this inevitably comes down to "well the 200 things I think are relevant do/don't work the same in favor of a/b" out of tens of thousands, if not more, of things implemented.

For me Firefox is about as good as any. Upsides here, downsides there, and nothing truly outstanding overall. It seems severely far behind in modern color (e.g. HDR videos or all of the wide gamut CSS specifications coming out), ahead in terms of single profile compartmentalization (containers), but behind in profile management (the profile manager sucks if you even know how to get to it). For as much as it champions being the last bastion of the free web it comes default with a decent amount of crapware (pocket, vpn) and has lost its way in terms of power user flexibility (UI customization, mobile extension limits, removal of power user flags). That said it's still better than the alternatives in those spaces, just not as good as it used to be.

For another none of these might matter but a lot of other reasons I don't even think about might. In the end even Firefox's own usage statistic point to the vast majority seeming to agree it doesn't have anything compelling enough to be worth switching to/evangelizing for anymore though.