XUL.mfl

The other day I posted a topic saying that all of my bookmarks disappeared (I have since restored them due to the nifty "bookmarkbackups"). That feature saved my life! Anyway, when I was backing up my portable apps, I received the following two eror messages:

Cannot copy XUL: The file or directory is corrupted and unreadable.
Cannot copy backup-6.session: The file or directory is corrupted and unreadable.

My first question, is what exactly is the role of the XUL.mfl file and why may it have been corrupted? Perhaps an unsafe removal of my thumb drive?

My second question, how can I restore the XUL file?