I'm not sure I understand the complaint. Feel free to make an issue on GitLab or email me if you want more help. However, this extension has no use any more as noted in the description of the add-on.

Originally when I created this, you weren't able to open Skype Web in Firefox at all. This was just made to bypass that. As I updated the description to indicate, this extension serves no purpose any more. I don't use Skype Web very often so I wasn't aware of the limited functionality message. I may look into that later, but I can't guarantee it's something a simple extension like this will be able to work around.

Yes, that is definitely why. I could release an update supporting that URL if you shoot me an email or make an issue on GitLab containing the address (links can be found here on the add-on page). I don't want users to have to agree for this extension to be able to modify their data for all sites, so I can't really do much else about this.