CSS transition flickering in Firefox

backface-visibility: hidden tends to fix a lot of flickering issues, try giving it a shot.

I had exact same issue: on several sites I've built that use CSS transform scale, there's a flicker the first time you hover over the images. Afterwards they're fine. Doesnt happen on any other browser, and is only recent - obviously a bug in later versions of FF.

Anyway, just fixed it by, of all things, altering the greyscale filter. Try this CSS on your img:

filter: grayscale(1%);

It makes no noticable difference to the colour, but the flicker is gone!