Drop-down menus sometimes prematurely collapse when mouse is between items on Firefox 4.01/5

On the Webpage http://musicbrainz.org/ (and others) in the main-menue,
most of the time it's not possible for me to click on menue-entries below the main-menue-entries. As an example: SEARCH -> Search Edits -> Browse -> Tags -> etc.. After the cursor reaches "Search Edits" and then a little bit below before the cursor reaches "Tag" the dropdown-Search-Menue collapses. This happens with other menue-entries too, but not so often.

(Edited by Chad:

This appears to happen when the mouse is hovering for more than x msec between one entry and another - I'm guessing there's 1px or so of dead space that is interpreted as being "off menu".)