The difficulty really ramps up quite steeply after level 11. It just kinda goes crazy. Also, the game should remember the sound settings each reset - it forgets currently, quite annoying.
Glitch with tabbing goes further than just firestorm. Buy 1 monkey, then tab to the upgrade button and press and hold space,. your money will go to 999999999 eventually, just buy all you want and get the badge,.. not hard at all.