Hmm, there isn't a width applied, but when I apply one, I still see the jump. Plus, it only happens on the first two menu items, not the rest.
Also, I can't set a fixed a width for these list items because they could change - the first item has a class "first" applied to it, but sometimes the first item has 5 characters and others it could have 15...
Im actually not seeing any jump
The only thing i can see is a very small gap between the dropdown and the list item itself ( but thats just because of the rounded corner. )
I'm only seeing a jump on the first navigation element. Your abundance of inheritance selectors in the related CSS make it rather hard for me to analyze, I can tell I would be there a while trying to link everything together.
Thought I would just say — and considering varied reports on the problem — it might be some default styling on the li, or maybe even the ul element. Y'know, like some tiny padding or something that one or two browsers throw in or mess with, that isn't meshing well with your hover styling. You could try zero-ing out or setting whatever you can think of that might move it.