How do I turn off or get rid of "Recently bookmarked" that are being displayed in my 3 line -> Library -> Bookmarks menu? I don't want them to be displayed.
I cannot find any way to disable, hide, or remove the Recently Bookmarked list from the bookmark section. These make this window unusable other than the bookmarks that are listed there. I have no use for Recently Bookmarked items to be listed anywhere. I have attempted all of "solutions" that pop up about fixing this issue, but none actually work.
There is no "Hide Recently Bookmarked" when you right click anything on that menu or others involving bookmarks. Similarly, there is no setting regarding it when you search in about:config as is claimed most of the time. Adding a bunch of meaningless bookmarks & deleting them, also does not clear the list as it seems to remember all the bookmarks.
I've found people complaining about this as far back as 49, and as recently as 66 and 78.
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All such 'recent' type list show the result of a database query and thus they will show results as long as there is one bookmark.
If this is about the Library toolbar button then you can remove this button via the customize window by dragging this button to the Customize palette.
Mine seems to show the latest week's worth of new bookmarks.
There is no built-in way to blank out the list there, but there are more useful ways to access your bookmarks:
(1) Optional Bookmarks Menu button for the main toolbar
Looks like 
and available in the Customize panel. See: Customize Firefox controls, buttons and toolbars.
(2) Classic top menu bar (File, Edit, View...)
Available using the Toolbars selector in Customize.
It actually is possible to hide menu contents using the community-supported (i.e., unofficial) workaround of a userChrome.css file, but perhaps if you have more organized access to your bookmarks using one of the above, then this isn't worth the additional effort.
Unsupported Workaround
Mozilla doesn't support hacking the user interface with a userChrome.css file, since continuous evolution of the interface can cause those "fixes" to stop working.
With that disclaimer out of the way...
If you are not familiar with userChrome.css, check out my website here: https://www.userchrome.org/
And the rule you would put in your file is:
/*** Hide Recent Bookmarks from menu > Bookmarks ***/
/* "Recent Bookmarks" Heading */
#PanelUI-bookmarks #panelMenu_recentBookmarks,
/* Recent Bookmarks List */
#PanelUI-bookmarks #panelMenu_bookmarksMenu {
display: none !important;
}