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.
You are experiencing this bug probably:
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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.
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.