Every so often drop down menus stop working in applications. This has happened in thunderbird and firefox. A quick fix is to restart the application.
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I have found that in my case this happens when I have used a windows program in wine while firefox is open. If I minimize and then restore firefox functionality returns. There is no need to close firefox.
Further testings shows this happens with a wine encapsulated program that allows minimize to taskbar when window close (X) is clicked. It does not happen when the window minimize button is used (_).
It would appear that the problem is that the wine program is not returning control to the firefox window when minimzed using the close button until focus is regained by re-opening the firefox window.
If a different ubuntu window is selected firefox functions again when you return focus to it.
The bug seems to be in the windows program, not firefox or ubuntu.
Do the reporters of the bug/problem use XIM input method(s)?
If so, you might want to check out the mozilla bugzilla entry
Using XIM with Firefox will cause firefox menu cannot popup correctly. https://bugzilla.mozilla.org/show_bug.cgi?id=787943
At least, one cause of the problem is traced to uninitialized timestamp in core X library.