First impression of the https://quillbot.com/ service: interesting.

When the extension's icon is moved from the toolbar to the menu (Waterfox), the user interface no longer fits. I imagine that the same problem will occur with the extension in the overflow menu of Firefox Quantum. For this, the faults are probably largely on the Mozilla side.

More unfortunately, there's no integration with the context menu for selected text.

The page at https://quillbot.com/ is more convenient (less volatile than) a pop-up. A page can be used with extensions such as PopUp https://addons.mozilla.org/addon/popup/ and so on.

For potential: three stars.

If the extension can be enhanced to work with the context menu: I'll give at least four stars for a future version.