Out of desperation, I followed a lucky hunch and disabled the extension "Ubuntu Firefox Modifications 2.1.1". That seems to have fixed the no-menus problem on multiple systems. So it seems to be a bug in the xul-ext-ubufox package--apparently it's incompatible with Firefox >= 14.
Out of desperation, I followed a lucky hunch and disabled the extension "Ubuntu Firefox Modifications 2.1.1". That seems to have fixed the no-menus problem on multiple systems. So it seems to be a bug in the xul-ext-ubufox package--apparently it's incompatible with Firefox >= 14.