Isn't it right in the address bar on a default Firefox install? If it isn't, then it's definitely in the 3 dot menu in the address bar.

It's in the right-click menu, and also in the the "..." menu on the right side of the address bar.

It's really easy to use. Just right click somewhere on a page.

Go to the web console (Ctrl+shift+t) and write :screenshot, ":screenshot --help" can give you a list of options to use.

It's in my right click menu.