Do you have a 3rd party popup blocker installed - perhaps as part of a 'toolbar'?
Does the popup blocker message not give any clue as to what the popup is? Do you have the option to show the blocked popup?
Assuming for the sake of discussion that this is shorthand for "even after I turn off the blocker, empty the cache, quit and restart the browser"? :)
Are there any other blockers, like ad blockers, lurking around anywhere? Any settings that involve new pages vs. new tabs? (In MSIE 7?!)
Do you get the message before, after or instead of the dropdown, er, dropping down? I'm wondering if the browser has somehow decided the dropdown itself is an evil popup that it has to block. Is it different in any way from the others?
Better post a bit of sample code, along with sample code for some random other dropdown that doesn't act up.
Post the code of menu as well as the files that you include in the head section of html page.