Rated 3 out of 5 stars
sort of works for me
Ubuntu 9.10 (Karmic Koala)
Firefox 3.5.7 (Mozilla/5.0 (X11; U; Linux i686; en-US; rv:188.8.131.52) Gecko/20100106 Ubuntu/9.10 (karmic) Firefox/3.5.7)
works fine except when I'm running unclutter (a background program that hides the mouse pointer when it's stationary). Tooltips seem to work in every other program with unclutter, although in those programs it looks like they get redrawn after the pointer disappears.