Bug 104778 - URI written in location bars doesn't persist tab/window switching (incomplete url is forgotten upon tab switching), which is one of the tabbed browsing bugs that has always irritated me, has been fixed on the Mozilla App Suite trunk/branch/tree/thing!
As one of the comments notes, Mozilla Firebird doesn't automatically get any of the Tabbrowser changes. So, hopefully it'll be ported over for Firebird 0.7.
I was just thinking about how much that bug bothered me yesterday. Good to know it's been fixed.