FireFox AdBlock and Flash
For a while, I've had problems with my flash plugin acting flaky in FireFox on Mac OS Tiger. I finally got fed up and tried upgrading to the Flash 8 plug-in. No luck. So I deleted my entire FireFox profile, down to nothing, and voilà, it worked again.
Then I started to add my FireFox extensions back in. Turns out AdBlock (which I love) was somehow interfering with Flash. I don't know if this affects Windows FireFox. It's annoying, because there are some useful Flash sites out there, notably the new Yahoo! maps.