If you're considering making the move to Firefox 3 (currently still in beta, but nearing full release), be aware that there are still lingering issues with Ancestry and the Enhanced Image Viewer. Currently, users are having difficulty with the plug-in, or, as in my case, are unable to install the plugin at all and are forced to use the Standard Viewer (which, in case you don't remember, really leaves alot to be desired).
Zotero is also not currently functioning in the 3.0b5 version, (though it did function in earlier versions). Zotero says it will be fully compatible with Firefox 3 when it is finally released.
Google Notebook remains buggy and unstable in 3.0b5, (and virtually unusable) though it looks to be an issue for Google to resolve.
I'll update as more issues come to light.