I am having the same problem:
http://wordpress.barbella.com/
Here’s a test blog
In Firefox, once hovering over a menu item such as "level 1".. the dropdown appears.
Then when going to move the mouse down to the menu… it disappears.
The behavior is odd, in that after a couple of movements, then the menu persists as it should.
What could be causing this?
I removed all the link borders, and minimized the CSS, and colored different objects to test what css controls which items. It seems to be a programatic behavior that Firefox is picking up