According to the latest poll within Mozilla, over 50% of the developers use a Mac. I suppose they're experiencing the same issues first-hand, and that it's only the matter of time before they iron out the performance.

It may do, but there's still a long way to go. Multi-touch gestures that use system animations (or at least approximate them), certain aspects of UI design made to fit the macOS aesthetic and workflow better, and other things. My personal hope is that once the newest UI implementation settles down, there'll be some more love added to it.

Stuff that Chrome got pretty damned fast, I hasten to add. Whether people like Safari or not, it does set the standard for browsers on macOS because, unlike IE or Edge, it's actually a very good and __very__ well-loved browser.

This has been said about a litany of releases, and they don't really ever make a dent.

I cannot remember the number at this point, but there was one specific release recently that did seem to make a significant improvement for me on MacOS.

I used to have heavy pages hang here and there and really burn some CPU (Facebook, etc) and sometimes even freeze without loading completely. Now on the same machine, the latest Firefox works just fine and avoids those states completely.

It's still a long way from perfect, but I once thought Firefox unusable on MacOS and went back to Chrome, but now that I've tried the latest FF releases, I'm happy enough with the performance that I'm staying with FF this time around

The only remaining sites with poor performance are Google products, and I blame Google for that and am trying to slowly remove myself from all of their products.