I pretty much run Firefox for most of my purposes. I found that Firefox 1.5 was very nice to use, but Firefox 2 is a disappointment.
Every so often they manage to break what wasn't broke, so I've been sticking with 1.5.0.6.
Get all your themes and extensions and tweak the browser the way you like it, and then get this program. (It might not backup all variations of themes and extensions, but it has for my particular setup).