Maybe this is not the addon I'm looking for, but I'm hoping maybe it could turn to be that.
Only tested it with music so far (from official label channel, so highest quality) available on YT.
- The RMS level dips quite a bit when I turn it "on" (I'm guessing turning up the GR);
- also it jumps at times when the dynamic range shifts - it makes me wonder if the gain compensation is set constant or how exactly.
- I'm using the Audio Equalizer addon from Muyor, and if both are enabled and this one is set to higher GR, I'm getting phasing issues in the sound, even in mono.

I hope the dev might further the code based on these and maybe - just maybe - we can have a great compressor or maximiser plugin in Firefox.