Shortcut to a manipulation of the current URL
Solution 1:
One extension that looks like it should work would be keyconfig. It appears like you can easily assign what would be a javascript bookmarklet to a key combination. So all you would have to do is do is come up with a bookmarklet that adjusted the URL.
- http://kb.mozillazine.org/Keyconfig_extension
- http://forums.mozillazine.org/viewtopic.php?t=72994
- http://kb.mozillazine.org/Keyconfig_extension:_Firefox