Ffx 3 has reached it's second beta and isn't far from full release.
I've had Minefield (as it was called during development) as a secondary browser for a while and now with the new beta version, I find it's stable enough to take over full time from Ffx 2.
What's new? Well there's some nice additions, the download manager is much improved and supports multiple session resuming (break the connection, close the browser etc. and you can still pick up where you left off). There is the option built-in to load-up with the tabs when you closed. The address bar now searches your history as opposed to finishing off what you're typing. Takes a bit of getting used to, but is very useful when you can't remember the start of an address but can remember the page name or subdomain.
Only downside is that currently a lot of extensions don't work, although these are likely to be supported pretty soon as updates are released.