Although it is not the only cause, this problem can be caused by a previous version of the Skype extension.
You can solve the problem by downloading the latest version of Skype from Skype.com. Make sure that when you install Skype, you click on the Option button down to the left of the first installation wizard screen. There you will see the option to Upgrade Skype extension for Firefox. Please make sure that that option is indeed checked.