Comment 28 for bug 106737

As for the "workaround": exit mozilla, rename that file on disk. but be
careful not to break any existing prefs or filters that point to that folder.

Whomever suggested this has never worked at a helpdesk serving non-techie types
who don't give a rat's a** what the underlying technical issues are. The
real-world non-techie user sees a silly, petty restriction -- and as was also
said "It works in Outlook".

If you have to first rename the folder twice, then so be it:
mozilla -> $$temp$$ -> Mozilla