Sometimes it just simply doesn't work. Hitting Ctrl+F9 just doesn't do anything at all. The hotkey is not bound to some other action as far as I'm aware (then it would do the wrong thing, rather than absolutely nothing). I have no idea yet when exactly it'll stop working, or what I'm doing at that time. It's not like I'm reloading css all the time, after all.
This problem started happening since, I dunno, today I guess.
I'm on Firefox Dev 59.0b3 64-bits on Windows 10.
Current workaround is to disable and enable the addon.