The issue has been the power draw and heat on newer MacBooks with a Retina display.

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.

I've been using Firefox on a newer macbook with retina display for the past 2 years or so. I haven't experienced "more or less unusable" in that time.

I’ve been using Firefox on MacBook Pros for the last 9 years, and it’s worked fine.