I have a retina display and I'm using Firefox right now with 5 tabs open. Working fine for me!
Do you use scaled resolution? It seems the bug is Retina + scaled resolution which is a default on some Macbooks.
150 tabs here, lol.
But it is true that for some users Firefox on retina is barely usable. It is a confirmed bug.
I think if you have a 2 core 2015 13" macbook it's slow. If you have a 4 core 2015 15" macbook or better then you are fine with firefox.
if you have a retina display, this "even" is sort of a joke, because ff is so CPU-heavy (and often slow) that it's unusable for me https://bugzilla.mozilla.org/show_bug.cgi?id=1404042