see if this add-on is helpful : Menu Editor add-on

thank you

Do you want to clear entries in both lists?

You can (temporarily) set the browser.sessionstore.max_tabs_undo pref to 0 to clear the "Recently Closed Tabs" list.
If you want to re-use the "Recently Closed Tabs" feature (Undo Closed Tabs) then you can reset the browser.sessionstore.max_tabs_undo pref to the default value via the right-click context menu.

• http://kb.mozillazine.org/browser.sessionstore.max_tabs_undo
• http://kb.mozillazine.org/browser.sessionstore.max_windows_undo

Close all tab in a window before closing that window to prevent the window from appearing in the recently closed windows list.

• This may not be possible with a pop-up window.

Set the browser.tabs.closeWindowWithLastTab pref to false on the about:config page to prevent closing the last tab from closing that window.

Firefox will remember one window if you open additional windows and close that window while tabs are open in that window.
To prevent that you need to close all open tabs in a window before closing a window.

OK. that clears it completely, but how do I make the clear recently history work so I don't have to change this setting. I use this something to return to web pages but there are times I want to clear it and the recently open windows tab too.

So how do I fix the real problem and get it to work the right way.

You can't use Clear Recent History to clear those two lists.
Those two lists are saved in the sessionstore.js file as part of the current session data.
Clear Recent History can only clear the browser history and other website data like cookies.

The only way to remove them otherwise is to open a new window and then close all tabs in the previously used window and then close that window like posted above.