Cover half of the link with a window. Then move your mouse over the link and before the tooltip shows, move the pointer directly onto the window. A second later, the tooltip for the link will show up. This doesn't happen in Firefox 1.0.7 (linux or Win32), nor the Win32 branch builds (haven't tested any other builds).
This bug happens on: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8b5) Gecko/20051007 Firefox/1.4.1 but has been occuring for quite a while.