I spend an inordinate amount of time using Firefox, both at work and at home. (No it’s not wasted time, most of my work involves online information.) My biggest beef with Firefox 1.5 was the incredibly slow and CPU-consuming performance when hitting a page that included Flash objects.
Then I found Flashblock. It blocks all Flash objects, and replaces whatever moving image might have been there with a nice F. If you really badly want to play the ad, you can click on it to run it. I highly recommend this plugin. Simply point Firefox here: http://flashblock.mozdev.org/ and click the “Install” link.