Hello! You can try the instructions in the following article: How do I delete a bookmark?

You can use code in userChrome.css to hide this specific content.

• [/questions/1222037] turn off recently bookmarked

You will have to change label names to their German equivalent.

Is this still working? The latest post is 3+ years old.

Most of this should still work. I think that the highlights are gone, hiding recent bookmarks and recent history should work. The label names are in html namespace and need a *|* prefix if you have the default @namespace line (...there.is.only.xul).

/* RECENT Lists - hide */
*|h2#panelMenu_recentBookmarks, #panelMenu_bookmarksMenu {display:none!important;}
*|h2#panelMenu_recentHistory, #appMenu_historyMenu {display:none!important;}

(edited to update the code)