I haven't tried firefox but use Mozilla. For me Mozilla is slow to load up at first but I can't notice much speed difference in page load times.
Some nice tips/tricks for Mozilla users:
. Get the preferences toolbar, with it you can disable/enable j a v a s c r i p t/flash/popups/cache/history/etc/etc with a single click plus many other nifty features:
http://prefbar.mozdev.org/
. If you want to search for something on a page, simply type '/searchterm' and you will immediately be taken to the first result. To jump to the next result click CTRL-G.
I love Mozilla and have no immediate plans to go back to Internet Explorer, mostly for security purposes.