You can't do that via about:config. You need to use code in userChrome.css.

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#panelMenu_recentBookmarks,#panelMenu_bookmarksMenu {display:none!important;}
label[value="Recent History"],#appMenu_historyMenu {display:none!important;}
label[value="Recent Highlights"],#appMenu-library-recentHighlights {display:none!important;}
#appMenuRecentlyClosedTabs,#appMenuRecentlyClosedWindows {display:none!important;}

You need to create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist.

• use a plain text editor (Windows: Notepad; Mac: Textedit) to create a (new) userChrome.css file in the chrome folder (file name is case sensitive)
• paste the code in the userChrome.css file in the editor window
• make sure the userChrome.css file starts with the default @namespace line

If you are on Windows:

• make sure you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userChrome.css

Otherwise Windows may add a hidden .txt file extension and you end up with a not working userChrome.css.css.txt file.

• make sure you do not have a double .css.css file extension (userChrome.css.css.css) if it is still not working

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

In case it helps, I have an article and video here about the userChrome.css file: https://www.userchrome.org/how-create-userchrome-css.html

Also, do you want to completely remove the Library button from the toolbar and use the Bookmarks Menu button instead?

The new bookmarks menu button () should work the same way as the old bookmarks menu button (), which was the "right-half" of the inseparable star+book duo.

To add that button, either:

(A) Use the Customize panel (Customize Firefox controls, buttons and toolbars) to drag the icon to the desired spot on the toolbar

(B) Use the method from Bookmarks in Firefox - section entitled: How do I add the Bookmarks Menu button to the toolbar?

• Click the "Library" button:
• Click "Bookmarks"
• Click "Bookmarking Tools"
• Click "Add Bookmarks Menu to Toolbar"

You have created a lot of separate questions about removing/hiding items in Firefox. Maybe consider to combine these in one thread to avoid repeating instructions.

• https://support.mozilla.org/en-US/search/advanced?q=&asked_by=omidsolo&w=2&a=1&sortby=1
• /questions/1198359 Synced Tabs
• /questions/1198355 Customize URL Bar
• /questions/1198447 Bookmarks Settings
• /questions/1198356 about:config key name

Can you give more detail what exactly you are trying to achieve? What kind of things do you want to remove/hide?