So after many many debugging sessions I finally found the real culprit for this issue here.. It has nothing to do with CSS, but with SVG. Who would have guessed?!
I figured — it has something to do with the web-font, so there is this issue and also that issue. But in reality it is some regression in Firefox.
Oh no. I have spent so many hours trying to figure out what’s going on. And now I am very sad to not really be able to fix this. My knowledge of SVG font rendering inside Firefox is very very limited, so I don’t feel like solving this issue anytime soon. Sorry about that.
Reading through the original ticket some high profile sites are also affected.. Hopefully this speeds up things..
Otherwise - this is something very frustrating - not be able to fix some bug. Let’s see what the future will bring..