I've been giving Firefox and Thunderbird a try for a few weeks now.
Firefox
I really really like the tabbed browsing feature. I do a lot of research on the web. I can search for something and then middle-click on each link to open that link in a tab. Reduces the browser clutter. And for bookmarks, you can put a bunch of related bookmarks into a folder and then with a single click, open all of them as tabs.
But...Firefox doesn't really work that well all of the time. It flat out won't handle some sites like MSN's investment portfolio plug-in, and seems fundamentally flawed when I use it to read some web forums like AVSForum. I rely on the AVSForum link on a thread to "take me to the first unread post". This pretty universally fails to put me on the right post using firefox. It always worked on IE. And some sites, for whatever reason (I guess the ABM crowd would say they were coded just for IE) don't work on Firefox. The visual formatting is screwed up or site navigation doesn't work.
Thunderbird
Thunderbird is the email, newsreading, and RSS aggregator app. Like Firefox, it has some cool features but overall the quality is missing the bar. I love the adaptive junk mail filters. It is getting about 90% of the 15 - 20 SPAM emails I get a day on my patjames.com email address. The RSS aggregation feature is crap. It doesn't hold a candle to Wildgrape Newsdesk, which lets me filter down to recent posts and includes all of the graphics from blogs like Gizmodo. And somehow it got into a funky state where I can't send email any longer (my SMTP server requires authentication to prevent SPAM relaying). I suspect an uninstall/reinstall will be required to fix it, probably losing my adaptive SPAM history.