Only partially supported at this point (FF and Edge (IE) have issues.)
If by Firefox, you mean Opera Mini, then yes. Opera Mini and Edge have issues.
Edge is in the process of testing this stuff, so this is likely to be coming soon(TM). If adding grayscale mode does nothing on Edge at the moment, I wouldn't be adverse to somebody working on this now. I'd be more enthusiastic about it if Edge supported it, but I don't think this /quite/ goes into "on hold." (Besides, I have much higher priorities than this right now!)