I was told you have already got the answer to other questions so here I discuss only this one:

 

c) Full screen modus from Firefox (under Debian Jessie) does not work. Is it normal?

 

 

Please explain what do you mean by "does not work". What are the symptoms? Please compare with other browsers, also in other OS. Is it the same? Please note that our "Full screen mode" is not literally the same full screen as, for example, web video players or photo gallery viewers. Our full screen is the window maximized and maximum chrome removed but some remains (e.g., the title bar, URL, your desktop's taskbar).