There seems to be a regression in Firefox 40.0.2 and its not loading the skype extension. When I updated to Firefox 40 the add-on stopped working,
The add-on seems to be enabled on the add-ons page but I don’t see the Skype icon in the top bar.
Here is a screen shot of some errors I see in the FF console when installing the extension manually - http://imgur.com/7Qi0oPd
The add-on is built using the add-on SDK 1.15
You can install the extension from here http://www.skype.com/en/download-skype/click-to-call/