More details from the Mozilla team:
The current shipping version of the Skype Toolbar is one of the top crashers of Mozilla Firefox 3.6.13, and was involved in almost 40,000 crashes of Firefox last week. Additionally, depending on the version of the Skype Toolbar you’re using, the methods it uses to detect and re-render phone numbers can make DOM manipulation up to 300 times slower, which drastically affects the page rendering times of a large percentage of web content served today (plain English: to the user, it appears that Firefox is slow loading web pages). We believe that both of these items constitute a major, user-facing issue, and meet our established criteria for blocklisting an add-on.
My response is "what took you so long?" I noticed a drastic slowdown when using the Skype Toolbar in Firefox years ago. I always disable it. If I really want to dial the number via Skype I can just copy/paste the number. The blocklist entry will be a “soft block”, where the extension is disabled, however the user is notified of the block can re-enable it.