i'm not the first to point out that everybody's favorite web browser and mail clients both released updates today. i've got to totally disagree with jonas here, i have to say that both the new versions rock.
maybe it's wishful thinking, but firefox 1.0RC seems to be a bit snappier on os x than 0.9 was. my only real complaint is that the incredible tabbrowser extensions don't seem to be updated yet. (i'll live). however, that extension allows you to put close buttons on each tab, re-order tabs by drag and drop (and a whole lot more).
the latest version also includes a funky RSS - bookmark thing. i'm not sure i really would ever use it, but when you're browsing a site that has an atom/rss feed there's a little "RSS" button that you can click to 'bookmark' the feed... which then creates a folder in your bookmarks that contain a list of headlines. nice i guess, though it ain't netnewswire ;)
the #1 reason i still love firefox though: the webdeveloper extension. it kicks serious butt. if you're unfortunate to spend most of your life building websites or web applications you should try it.
thunderbird 0.8 brings one big new improvement (as far as i'm concerned) and that's the ability to quick search by different fields in a manner similar to mail.app. the other feature (on os x) is that the dock icon now shows a count of new messages (instead of the green checkmark) - though i wish it was persistent a la mail.app.
i still have some nit-picky complaints with thunderbird, namely i wish that you could quickly limit the message view to all 'flagged' messages (on IMAP accounts) or if the thunderbird markings were stored on the IMAP server (rather than the local cache) so that they persisted across machines. the only other thing is that, i agree with jonas here, not doing addressbook.framework integration is stupid on os x, though it looks like there is a patch.
i was a long time mutt user... and still use it on occasion. but mutt's imap support sucks (no local cache makes imap unfun for lots of mailboxes & messages). and, it's _really_ hard to compose multiple messages at once on a text mode mail application.
i'll still stick with the mozilla* family. the apps are good - but the #1 motivation for me is that they provide me (in combination with gnu emacs) a truly cross-platform solution for my "mission critical" applications.
... now if only there were a linux equivalent of ecto. ;)