Like it says. I tried importing preferences into Tab Mix Plus, but when I clicked My Documents in the open dialog it showed a blank space. I went to Windows Explorer and saw that when I clicked My Documents I was redirected into Firefox Portable/Data/userprofile. I use the FirefoxPortable.ini file to map userprofile to that folder, as specified in the Firefox Portable support page. I'm not sure what exactly caused this, but the only thing I was doing at the time was installing extensions and configuring a newly downloaded Beta 2 Portable.
Deleting the Firefox Portable folder (it was on my hard drive) remapped My Documents, but it made the link in My Computer to My Documents disappear (the one that says Name's Documents). I did a system restore and everything's back to normal now. I'll try and reproduce this if I can. I know betas and such aren't supported, but I thought this was a novel bug, so I posted anyway.
Happened on Windows XP Pro SP2, using Firefox Portable 2.0 Beta 2.
EDIT: Reproduced on another computer running XP Home SP2. I downloaded Firefox 2.0 Beta 2 Portable to the desktop. I extracted it to the desktop and opened it. As soon as my homepage opened, I clicked "Save Page As...". I clicked on My Documents, but did not save, and closed the save dialog. Exited Firefox Portable, and the drive was remapped. My Documents links on the desktop or in the start menu are unaffected, as these appear to be like shortcuts pointing to the folder in Documents and Settings. The "Name's Documents" folder in My Computer is affected by this bug, as are My Documents references in open or save dialogs. I tested the bug both from an extraction on the computer's hard drive and on my flash drive, and it happened both times. I also tested it with and without the .ini file. Also happened both times.