> I take screenshots using the Firefox feature sometimes, but never with the right click option.

Um … how else do you access this feature?

(I use the context menu's item for that all the time … since that's the only way at it that I know of.)

You can add a screenshot button to the toolbar from the "Customise Toolbar" screen, does the exact same thing as that context menu item.

By pressing the context-menu key when the element is selected in DevTools.

Ctrl-shift-s