I have seen good results with Firefox if I disable smooth scrolling. This function eats a little CPU.
Chromium also ships without smooth scrolling.
Check for updates and install the latest Firefox. Now I'm running version 28.0 and I have no problems with flash. If I'm playing some flash games it is true that chromium will give you some extra fps, at the cost of more memory being used. Delete cache once in a while and Firefox will breath a little easier.
And lastly disable any Add-ons that you don't use ...
PS: Happy browsing! And HTML 5 is much much faster