"There were a ton of issues using Gecko, starting with (at the time) no CDM (HTML5 DRM module) so no HD video content from the major studios, Netflix, Amazon, etc. -- Firefox had an Adobe deal but it was not transferable or transferred to any other browser that used Gecko -- and running the gamut of paper-cuts to major web incompatibilities especially on mobile, vs. WebKit-lineage engines such as Chromium/Blink."
And they're the only browser that has a functional alternative for webpage-based ads. Active right now. And you can instead fund pages / creators by buying BAT directly instead of watching private ads.
On top of that, Brave's defaults are much more privacy-protecting than Firefox's, you only get good protection on Firefox if you harden the config by mucking about in about:config.
People love to hate on Brave because they made some weird grey area missteps in the past (injecting affiliate links on crypto sites and pre-installing a deactivated VPN) and they're involved in crypto. But its not like Firefox hasn't made some serious missteps in the past, but somehow Firefox stans have decided to forget about the surreptitiously installed extension for Mr. Robot injected ads (yes really).
If people could be objective for a second they'd see that Brave took over the torch from Firefox and has been carrying it for a long time now.
[0] https://brave.com/research/sugarcoat-programmatically-genera...