With regards to Firefox... I have zero problem, and rare, in 10 yrs using it.
Upgrade to the latest build, works better with 10.6.4.
Inspect what you've added as far as extensions and plugins.
I would hope that export bookmarks isn't too trying!
I use Firefox to sort and manage bookmarks (non of the bookmark managers are perfect for my needs).
Force Firefox (and or Safari) to start fresh: remove all the cache, temp files, history and index and prefs (just leave bookmarks unless those are corrupt).
~/Library/Caches - Safari is in "Metadata"
~/Library/Application Support/Firefox
http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/
I use NoScript, Adblock Plus, FlashBlock only and it is rock solid and faster than any browser (though I don't care for Google's and haven't tried 6.0)