Note: You must access about: protocol pages by typing them into the address bar. Attempts to navigate through window.location will throw — Error: Access to 'about:addons' from script denied.
Well, I guess you could probably use document.execCommand to do it, unless that is blocked for JavaScript uri scheme access, or ask for async clipboard access (I wonder what happens if you ask for it from JavaScript uri, should take time to investigate some day)
at the bottom
Note: You must access about: protocol pages by typing them into the address bar. Attempts to navigate through window.location will throw — Error: Access to 'about:addons' from script denied.