We don't have an Intel-based Mac yet, though. I haven't noticed anything out of the ordinary with FF compared to on Winders.
Apple's "Mail" seems a little weird compared to Thunderbird, but it works fine. Both are better than Eudora, IMO.
You might want to look into [link|http://www.finkproject.org/index.php|Fink] if you like the idea of installing Free/Open Source software on OS X.
Have fun!
Cheers,
Scott.