Personally speaking, the three things that matter most for me as an ordinary end user are speed, privacy, and - when on a laptop - battery life. The first criterion means Safari is out of the running, as its webkit engine just doesn't perform as well on my MacOS devices (which, admittedly, both still have an Intel chip--maybe it's different on Apple silicon). Firefox is actually a little bit faster than Brave on desktop, but the problem with Firefox on MacOS is ...
Battery management. It just gobbles up my Macbook battery compared to Brave and seems to do a much poorer job of throttling down background scripts and other kinds of background connections. To address this, you basically need to install a special Firefox add-on called superstop, but I personally don't like having to nurse my browser like this.
In terms of privacy, both browsers are good, but Firefox defaults users into their telemetry, which supposedly goes toward "anonymized studies." Not sure what that is, but no thanks. I shut that off, just as I shut off Brave's crypto and BAT stuff.
But probably the biggest deal breaker for me in terms of using Firefox as my primary browser is that Firefox on ios is a hot mess. Really, really bad. And I'm not talking about just visual design, which is just a matter of personal preference. I mean, it has no built-in comprehensive ad blocking (which even Edge on ios has, as idiosyncratic as it might be). It claims to block a few ads that can be used to track, but I've seen no practical pay-off to that.
As a result, Firefox on iOS is slooooooooooow. It makes my ipad heat up just loading a couple of pages, which takes maybe twice or even three times as long than loading the same page in Brave on ios. Firefox on iOS also crashes constantly, at least for me. Since I rely on cross-device syncing, the tragedy that is firefox on ios devices is kind of a dealbreaker and I have been using Brave for many months now.