I've been using Avant, http://www.avantbrowser.com which is basically a tabbed browsing wrapper around IE.

I've played with Opera, which I like, except that the free version takes up a bit too much screen real estate for me.

I've played with Mozilla, which I like OK under Linux but really don't like at all under Windows as it takes forever to de-iconify if you have a small memory footprint.

Firebird is at release 0.6 or somesuch, which seems a bit too early for me to adopt just yet.