Seamonkey 2.3.1 - never been a big fan of Firefox. Stayed with the Seamonkey 1.1 branch for quite a while as early 2.x releases weren't great.
Firefox 3.6.21 - having jumped over many versions that caused problems. Have Live HTTP Headers installed here, as not yet compatible with latest versions of Seamonkey.
Also Opera, Safari and K-meleon used from time to time. Chrome also installed and used perhaps twice in the last year. IE is installed and never used. <edit>Just uninstalled Chrome; don't need it.</edit>