if you have a invalid path in your signature mozilla seems to create the path! Reproduce: Enter "C:\test" as signature without having a file called test or a directory called test in C:\ When you press Ok in the Account manager mozilla creates the directory "c:\test"
This sounds awfully familiar to me... there might be a duplicate out there.
probably a dup of http://bugzilla.mozilla.org/show_bug.cgi?id=23248
*** This bug has been marked as a duplicate of 23248 ***