What I do is to have Flash enabled in IE7, and I use the excellent Flashblock in Firefox, which is the browser that I use most of the time. For the odd site that needs flash and that Flashblock interferes with I can use IE, for everything else Flashblock gives me control over Flash

http://flashblock.mozdev.org/

Aside from these idiotic Flash ads making people's computers unusable, because of their incompetent programming, think of the amount of extra electrical power being wasted with those completely unnecessary CPU cycles. It's bad for the pocket and bad for the planet