Wait a few days, weeks, or months.
Fire up Firefox again. Hit ⌘-T. You'll get a new tab and focus will be placed…who knows where? It's damned sure not in the address bar where it belongs though.
This has happened twice now. Once when I got the Mac and recently since I rebuilt my environment.
Does anyone know why? Or what I changed? Or, best of all, how to fix it?
[Update 22 Jan 2008] Jason pegged it. It's not one extension causing the problem, though, it's two. The problem seems to arise (only?) when VeriSign's OpenID “SeatBelt Plugin” and “Tab Mix Plus” are both enabled.
[Update 24 Jan 2008] The Tab Mix Plus folks devised a patch.