Skype's annoying Firefox extension doesn't have a proper uninstall option, but you can get rid of the blighter with a bit of simple hacking.
I've complained before about how the Skype Firefox extension renders most numbers you encounter in your browser meaningless, but despite having promptly disabled it, I only recently decided to get rid of it altogether. That seems challenging, since the Uninstall button on the Add-ons menu is greyed out.
Fortunately, regular Lifehacker US contributor the How-To Geek comes to the rescue with a method for getting rid of it. In a nutshell: you need to delete the relevant folder from the Firefox extensions directory. Hit the post for the full details.
Manually Remove Skype Extension from Firefox [How-To Geek]