The items in Firefox/Services menu disappeared after I restart my iMac, but not in every browser tabs. For example, they are there and working correctly if the browser display the Help/Troubleshooting page (I'm not sure the name is the correct one, I have a localized version in Italian: "Aiuto/Risoluzione dei problemi").

If I actuate Firefox restore function, Services work properly until I restart the iMac.

I tried to run Firefox with the option "-NSDebugServices com.apple.mail" getting the following messages:

Mail/New Email With Selection (com.apple.mail) is enabled in the services menu and disabled in the context menu, by the standard Services policy.
Mail/New Email To Address (com.apple.mail) is enabled in the services menu and disabled in the context menu, by the standard Services policy.
Mail/New Email To Address (com.apple.mail) is disqualified because its send and/or return types cannot be handled by the requestor ChildView 0x11b6e4680, gecko child 0x11ba56800, frame {{0, 0}, {1580, 1040}}.
Mail/New Email With Selection (com.apple.mail) is disqualified because its send and/or return types cannot be handled by the requestor ChildView 0x11b6e4680, gecko child 0x11ba56800, frame {{0, 0}, {1580, 1040}}.
GVA info: Successfully connected to the Intel plugin, offline Gen9 
WebGL(0x12dc38000)::ForceLoseContext"

Please, do you have any idea how to fix my installation?

iMac Retina 5K Late 2015 - OS X 10.11.6 - Firefox 50.0.2

After the Firefox 50.1 release updated "The song remains the same"... but: