Wil Wheaton, of Wesley Crusher and 'Just a Geek' fame, but latterly known for his popular website, has had nice things to say about Firefox. You can find my thoughts about the latest builds elsewhere on this site.
I also commented on Wil's site: To those folks who have had problems with firefox (e.g. version 0.8), remember that it is still in beta release.
It's now up to 1.0 (preview release) and they're finally hammering all the bugs (though I find more in MSIE).
I find firefox to be rock stable. It's great. Bugmenot is a wonderful extension, I also have the gmail plugin, web developer, HTML validator (lovely), single window and ieview (which hops a page to MSIE - I only ever use this to check that MSIE isn't doing something silly with a stylesheet).
I did have some problems with an earlier build of Thunderbird, but this too looks stable now. Both thunderbird and firefox have good support for RSS feeds - personally I think that the thunderbird model for RSS feeds is better (the RSS updates and it looks like new email).