Probably after upgrading FF to 55 (64bit) on Windows 7, all pull-down items get disabled a while after FF is started. The affected items include not only the web page pull-down menus but also FF's own menu bar items, tab browser (the tiny downward triangle far right to the tabs), "Open menu" button (the three thick horizontal bars rightmost in the tool bar), etc., almost everything. Some web page pull-down items still work, but they are probably in different CSS/JS/HTML coding. The menu bar shortcuts (e.g. alt-T for Tools) become unresponsive, too. Everything works fine right after FF is started, but, a while after, varying from a few minutes to a few hours, all FF windows start suffering this symptom simultaneously. Restarting FF resumes the good, but the symptom returns a while after. Starting FF in Safe Mode makes no difference; neither does restarting OS. User activity on FF (e.g. typing and clicking into FF) does not seem to affect the symptom; leaving FF untouched at all for a while after starting FF also leads to the symptom. No other applications running concurrently on the same PC suffer this. 55 (64bit) on Windows 10 works fine.