The Skype Toolbar for Firefox is an extension that detects phone numbers in web pages, and re-renders them as a clickable button that can be used to dial the number using the Skype desktop application. This extension is bundled with the Skype application, and is installed into Firefox by default when Skype is installed or, in some circumstances, updated. As a result, a large number of Firefox users who have installed Skype have also installed the Skype Toolbar, knowingly or unknowingly.

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.

As a result, we’ll add all versions, up to and including the current shipping and beta versions, of the Skype Toolbar to the Mozilla Firefox Blocklist for all versions of Firefox. The blocklist entry will be a “soft block”, where the extension is disabled and the user is notified of the block and given the option to re-enable it if they choose. It’s also important to note that the Skype application itself will continue to work as it always has; only the Skype Toolbar within Firefox is being disabled. The blocklist entry will be added sometime in the next 48 hours, and will be in addition to the existing soft block on earlier versions of the Skype Toolbar.

We’re tracking the issues we’re seeing with the Skype Toolbar in bug 615799, and the blocklist process in bug 627278. We’ve recently established contact with the Skype Toolbar team, and will work with them to identify the issues that should be corrected, and will lift the soft block on future versions that address those issues.