Skype and Firefox haven't really been on the same page lately. The Skype toolbar that was developed for Mozilla's popular browser has been causing a ton of problems for users with a reported 40,000 browser crashes in the last week alone. This has caused Mozilla to put a blok on the Skype toolbar while they figure out what the problem is.
The toolbar mostly allows easier Skype usage by identifying phone numbers on web pages and pushing them straight to Skype for easier call making. This is being referred to as a "soft block," which means Firefox is disabling the toolbar by default unless you go it and turn it back on; putting you at risk of the headaches a ton of other users are having. Just one more reason to avoid toolbars altogether.