Have you considered using the keyboard commands for Zoom?
https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

Or use {Alt} to show the Menu bar - then hit {V}for the View menu - then {Z}for Zoom - then use {I} to zoom in or {O} to zoom out from the keyboard.

Adding menu items isn't easy, plus the internal commands for the new menu item would need to be coded. You would either have to locate an extension which adds those menu items or learn how to create an extension that has that feature.

I did consider it, but there is no way I would remember the commands. I know how to zoom in and out with the mouse, but you can't restore it to the normal 100% zoom that way. And I don't want that hidden menu to show up (plus I would forget the keyboard commands for that too)

Tapping the Alt key to show a menu bar is becoming a standard in Windows applications lately. Internet Explorer, Windows Media Player, others I can't remember, use this key. Traditionally, the F10 function key also has been used to activate the menu bar on Windows.

Not to say you shouldn't explore possible modifications to the button using an extension, but looking ahead 6 months, Firefox is slated to delete the orange button in a future redesign. I wouldn't spend hours and hours on it...

Then see this - https://developer.mozilla.org/en-US/docs/Building_an_Extension

See also:

• Personal Menu (Personal Firefox Button): https://addons.mozilla.org/firefox/addon/personal-menu/