Hi, s20
Thank you for reviewing.

> - When the icon is removed from the toolbar and "overflow menu", the extension does not work.
Surely, this extension does not work if it is removed from the toolbar. This is specification of Firefox.
If "overflow menu" means an icon in ">>" of the toolbar, it works for me.

> - It is impossible to select text inside the popup.
Yes. Sadly, almost all text cannot be selected in the popup.

> - It is impossible to navigate back and forward inside the popup.
Yes. Sadly, Firefox does not support these features.

> - When used with the "Open in Sidebar" extension, both display the same text in context menu when set to open the links without asking:
I had an idea like this but I have not added it. Because a selection which is added "in sidebar/popup" is redundant. The icon in context menu tells what extension will be used.

> The choice between http and https is redundant and confusing, today mostly everything goes via the https protocol.
Yes. That's why I let an user choose https in the tutorial.

> Also, a monochrome icon more akin to Firefox's native icons may be an aesthetically better option.
It may be.

If you have additional questions or feedback, please submit new issue in github or send me an e-mail.