Most probably firefox.exe wasnt switched off by the time you started next firefox .. it sometimes happens.
Now you can use the
Profile Manager to change your profile details.
Or you can
Migrate most of the settings to new profile
BTW, next time this happens .. Cancel it .. dont start firefox for .. say 2 mins .. wait till you're sure FF is gone. Then start it.
EDIT: I'm not sure .. but most probably you can do the following to revert back too.
# Go to the Firefox's Profile folder ... which is
For WinXP :: C:\Documents and Settings\<Windows login/user name>\Application Data\Mozilla\Firefox\Profiles\
For Win98 :: C:\Windows\Application Data\Mozilla\Firefox\Profiles\
# You will see 2 folders with weird 8 letter names ... one being "********.default" and other "********", where "********" is some random sequence of letters.
# Now, rename the "********.default" to "********"
and rename "********" to "********.default"
I think this should change your default profile back to normal !!
But I'm not sure ...
Change back to original config if this method doesnt work
-----
alibi