This is another scrollbar bug that I don't think I've seen filed yet. Sometimes when I resize the Messenger window, the scrollbar doesn't change position and it looks like it's floating in the middle of the window. I find that when I use it, it suddenly attaches itself to the right side again. I don't know how to reproduce this, but I've seen it a number of times.
This may be similar to bug http://bugzilla.mozilla.org/show_bug.cgi?id=10426 reported by Laurel.
Marking dup of 10426. *** This bug has been marked as a duplicate of 10426 ***
marking verified as a duplicate.