This could be an amazing game, but you need to take into consideration quite a few aspects.
Firstly however congratulations on the 3d aspect, I can imagine that was a real pain to make. But seem to have rushed the rest of the game after this, I've pointed out a few problems below:
1. Immensely fast reaction times.
The time the player is given between when the enemy appears and when he starts shooting, is nigh on instantaneous. People playing flash games, may not have extremely sensitive mice and may not be gaming pro's, so this can make for an impossibly difficult game. (solution: try introducing a random timer before the people shoot)
2. Instantaneous appearance.
This kind of bugged me, the characters appeared out of no where, just flashing on at the designated location. I can appreciate animation is not for everyone, and it does take time, but consider downloading and using a tweener class, and setting up an inTransition() function for you enemies to use to get them to their designated area.
3. Reload cover?
What I think is cool in a scrolling fps is when you can hold reload and you actually take cover for a short period of time, this enables give the player a chance to think and update their strategy. Again this would involve additionally animating moving behind cover, but I think the return is well worth it.
Self explanatory, a few health and ammo pick ups would be useful!
All in all not a bad game, going by the first review I hope this doesn't get blammed, but If you want to make this a hit I really think you need to think about making the game more player friendly.