FYI, I had to go finding this the other day - it's tricky as mousing out removes the class. Anyway, it's:
.chzn-container .chzn-results .highlighted .chzn-container .chzn-results .highlightedIn Firebug, you can ask the :hover state to stay active.
1. HTML tab
2. Click on the element
3. Style menu on the right panel
4. choose :hover
It tricks the browser into thinking the mouse is really over the element (i.e., it doesn't just add :hover), so if you have a hover class added via JS that should work too.(Apparently this feature has been there for a long time, but I only found it recently. Changed my Firebug life.)