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Mozilla announced yesterday that it will block the Skype Toolbar add-on for Firefox and remotely disable it for existing users. Mozilla was forced to take this extraordinary measure after discovering that severe bugs in the add-on are crippling the browser's performance and stability.

The Skype Toolbar add-on is developed by Skype and comes bundled with the company's popular chat program. The add-on appears to be injected into Firefox automatically during the Skype installation and update process. Its primary function is to identify strings of text in Web pages that look like phone numbers and transparently convert them to links that can be used to automatically dial a call with Skype.

In November, a Mozilla engineer noticed half a dozen reports in the Firefox bug tracker that involved problems caused by the Skype Toolbar. A meta-bug was established to track the issues collectively and facilitate discussion about potential remedies. Mozilla's crash report system also identified the Skype Toolbar as one of the leading causes of Firefox crashes.

Some of the problems that Mozilla uncovered are fairly serious. The toolbar apparently performs its phone number conversion routine after every single DOM mutation, severely impairing the browser's performance. In some builds, the performance hit is so bad that it makes DOM manipulation 300 times slower. The add-on's misbehavior also reportedly causes rendering problems in a number of scenarios.

ARS Technica has thee details HERE!