For as long as I can remember, I've had this problem where firefox sucks up 100% of my CPU, freezes, locks, effectively dies but won't go away... and I have to kill it. I've always suspected it was Flash, but now I have some form of proof.
Installing the Firefox flash blocker extension made the problem go away. Now on sites where I want to see the flash (e.g. flickr) I can just click on any flash I want to see. On other sites, it doesn't start it up.
No more 100% CPU. No more locks. Eric is a happy man.
Also, in the debugging process, I discovered that pressing 'Escape' will stop all animated GIFs on the current page.
These two things combined mean that my browsing experience has dramatically improved in the last few days.