Excellent game!
I'm a fan of defence games, and this was definetely a good one. I thought it was a bit more balanced than the first one too.
However, I also had the browser freeze at level 25. Perhaps you might want to consider adding some more critters with even more health, or even shoot back perhaps? Just so you have a different option than just adding LOTS and LOTS of critters. The last few levels I was playing at 6-7 frames per second.
Overall, a good game though!