drag & drop a link from the URL line into a folder on the personal toolbar. hover there until the folder opens. in some instances (apparently when the folder has subfolders), this results in the "paper" icon being left behind the newly opened drop-down menu, which will not close. parts of the mozilla UI become unresponsive (but clicking on links in the now-open folder works) and the only way to return to a fully-useable mozilla is to switch to an xterm and issue a kill command, because one of the functions that stops working is closing mozilla. my desktop is set to focus-follows-mouse, not the (apparently more common) click-to-focus, which may or may not contribute to the problem.
This is a dupe of Bug 96504, "after dragging proxy icon onto bookmarks button, bookmarks menu refuses to close". A temporary fix has been applied to recent Linux/Unix branch builds, the Bookmarks folder on personal toolbar does not open when hovering.
*** This bug has been marked as a duplicate of 96504 ***