Yawn, sounds like you should try to update and reinstall Flash (Though I have to admit, I wonder how that would make the browser crash). It may also be a problem with your browser (Update them). And update your system to the newest.
It may also be that your computer is too slow and old for Gemcraft 2 (It is a pretty big game). Basically all computers with less that 1 GB processor will experience problems with the game.
Lower the quality.
Look for signs if your browser is going incredibly slow, or the system. If that's the case, do what Axel says.
Maybe your system is indeed to 'small' for this game to run smoothly... don't know of any other reason possible?!
Only thing worth mentioning is that I get a lot of Adobe upgrading reminders lately... It keeps popping up with the message telling me there's a new Adobe Flash Player function available, but is it really? :S
