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.
Firefox has image rescaling that doesn't blow chunks at non-integer scales.
For example scaling up the page 120% to be able to more easily read text that the designer made too small doesn't turn all the images on the page into a blurry mess like it does on Chrome.
I've tried reporting bug reports to the Chromium project, but they can't seem to understand the concept and keep misfiling the reports as duplicates of irrelevant bugs and closing them as fixed without having fixed the problem.
Constant crashes that lose open tabs, layout glitches where display gets offset from ui trigger points (including on this site), downloads that just fail / don't work.
You'd think that this would have me stop using it, but I really don't want to use Chrome/Brave/DDG so have just been putting up with it.
Like most small-backing Chromium clones it's usually ~2 months behind on patches (normal and security) and the published source is about ~10 months behind. Being effectively closed for the release version and very small from a development side it does wonky things like redirect your "Google" searches through their servers since they don't have proper search agreements.
I.e. it's basically "an interesting Chromium clone maintained by a couple of folks out of Estonia". If you're cool with that it can be a nifty browser, for many that just makes it to unproven to dump access to their entire online world into.
i'm using it every day along with Safari and don't see any significant differences