If you want to suggest a feature for the add-on, please do so on GitHub at https://github.com/rugk/offline-qr-code/issues and open a new issue. (Particularly Thunderbird support, which may or may not be possible, but it's an idea.)
Regarding the color context menu entry: This is consistent with how other add-on do it (and unfortunately not really possible to change without changing the whole add-on icon) and follows Firefox Photon style. Also note you can toggle the colored popup icon in the settings, but it's a different icon.
