On other peoples computers mind you :-P
Basically just kill all firefox.exe processes, and start firefox.exe with the -p syntax to get into the profile manager, and select the profile named "default". :-)
Apparently, a new instance of firefox can't access the default profile sometimes when another firefox process is using it, and thus creates a new blank profile instead. At least, that has been the issue the three or four times I've encountered the problem.