I was able to get it to load on Edge of all things, so I have actually played it now. Not sure why it refuses to play on Firefox.
It seems like no matter which colour you play as or which direction you hit somebody they always move to the right, and since all stages are fought on one plane the ability to jump is moot. Strengthening the gameplay's skeleton is direly needed. Some verticality in the level design so that jumping is useful, and probably some more attacks so there's even a base element of strategy to playing beyond hammer the punch button as fast as possible. The problem there will likely turn out to be keyboard real-estate, since a lot of keys are already assigned to various potential players.
This might be a reasonable learning experience, but maybe it would be beneficial in the long run to learn how to implement AI opponents for this kind of game so that the next project can launch on a more solid foundation.

Its ok...
but you need to assign different controls for players (more intuitive and so that it doesn't interfere with other players)
and add some music
I can help with music if you want