Shoutbox

To get all profiles unlocked, go to:
C:\Documents and Settings\<username>\Application Data\Mozilla\Firefox\Profiles
(for non-XP machines, vary accordingly)
then go into each profile folder with Firefox fully closed and delete any lock files. I can't give you the name of the one to delete, as my Firefox is set up slightly differently, so it only uses one profile.

On Linux, the file is '.lock', whether that helps at all.