The current version of Firefox on macOS is a vast improvement over what Mozilla was shipping 6-12 months ago.
However, it still requires more RAM, doesn’t render as quickly and consumes more power than Safari, so there’s that. Also, Safari implemented anti-tracking features before Firefox did.
This is a gross exaggeration. I've been using firefox on MacOS for 3 years and it's worked fine the entire time.
Older Firefox versions are redrawing the entire window even if only a tiny portion of the window was changing.
With advent of high resolution windows, the amount of work needed to redraw the whole window for every frame became significant.
The last version of Firefox moved to breaking the window into large tiles and only redrawing the tiles that have changed.
This is still less optimized than Safari (which uses Core Animation to only redraw the items on the page that are changing) but it is still a large improvement over prior versions of Firefox if you have a high resolution display.
Do you consider Firefox to be a less-than-great product?
I use it as my main browser, from Windows 10. No complaints. I don't experience performance issues or UI issues. Chrome and Firefox are both solid browsers, on Windows at least.
Perhaps Firefox for Mac is a weaker product, and loses to Safari 'fair and square' on quality grounds?