Headless mode maybe, but for regular users I don't think there's many issues.
I'm running firefox as my daily driver on Mojave with no issues whatsoever. Without seeing this thread, I didn't even know there were issues with the mac version.
Just to add to the pile of anecdotal evidence - same here...I switched to Chrome some years ago when the speed difference was just too much to ignore and switched back to FF a couple of months ago after I started realizing the growing firmness of the cold grip Google has on the web more and more...There's absolutely no noticeable speed difference any more, fans on the laptop are as always and it's been smooth sailing since (on Mojave). The only thing that still annoys me when developing is the lack of websocket frame inspection - but as a "regular" user, no issues at all.
I'm running firefox as my daily driver on Mojave with no issues whatsoever. Without seeing this thread, I didn't even know there were issues with the mac version.