The browser plugin, which automatically installs itself to all Firefox user profiles upon installation of the Skype application, lets the the users call directly to the numbers displayed on websites using the Skype application.
All versions of the Skype Toolbar for Firefox will be blocked in all versions of Firefox.
Update from Skype via TechCrunch:
Based on our initial investigation, we know that downloading the new client will fix for most users any compatibility issues, and we are working with Mozilla to ensure that there are no other compatibility issues. We are sorry for any inconvenience this has caused our users.
Users can download the latest Skype client with the latest included Toolbars OR the latest toolbar installer itself is here.