I've been using the Firefox browser since it was firebird...it's pretty good and has all kinds of neat adins but there are some problems.
1. Java and external apps: On a fresh install of firefox, it still has some problems with java hooks for apps in browser. I mean it's a lot better than it was before but still sort of sucks. External apps with plug-ins sometimes do not work correctly due more to the construction of the website then the browser but it's still a pain.
2. Security: there have been a couple exploits already, until recently you basically did a deinstall of you browser and had to install the new version, losing all the ad ins..not so bad anymore but still the best ad ins take a while to catch up. So if you upgrade to fix a security feature, you may not be able to use your ad blocking software for a week or so.
3. Presentation: If a page looks good in IE and Netscape, it still doesn't always look good in Firefox.
4. Security SSL and protected websites: There is a fundemental "flaw" in the way Firefox trys to "trick" websites to make them think that it's IE or Netscape for sites that only "support" IE/Netscape. This is a catestophic problem when it comes to SSL and banking sites.
Thunderbird:
No one has proved to me yet that it's superior to other mail clients. In fact if you do any httpmail you can't do it with thunderbird, right there alone...it's not worth it for me.