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.

A Skype employee who joined the discussion in the bug tracker earlier this month offered to bring the issue to the attention of the relevant Skype personnel. Despite that and similar efforts by Skype employees to get the right people involved, Mozilla had enormous difficulty getting into direct contact with the team at the VoIP provider that is actually responsible for the toolbar.

After spending nearly two weeks trying to get somebody at Skype to commit to fixing the problem, Mozilla's Justin Scott finally decided on Wednesday that imposing a block on the add-on had become necessary. In a message posted in the bug tracker, he pointed out that the toolbar had single-handedly caused over 33,000 Firefox crashes in the previous week alone.

"Given the volume of crashes, the extent of the performance impact, the fact that users don't actually choose to install this add-on, and the extension team's lack of response during the weeks we were giving them to solve these issues, we should continue as planned to soft-block all versions of Skype in all versions of Firefox immediately," he wrote. "If these issues are fixed in a future version, we will be happy to reduce the block to only the affected versions. But with 33,000 crashes in the last week, this can't wait any longer for a fix from Skype."

The "soft" block will remotely disable the add-on, but allow users the option of manually turning it back on. The block will be imposed on all versions of the toolbar. Mozilla has previously imposed soft blocks on old versions of the add-on that were known to be problematic, but this appears to be the first time that they have blocked the latest stable version. It's important to note that disabling the add-on will simply sever Skype's browser integration—it won't damage the Skype application itself or impair regular usage of Skype. Mozilla intends to work with Skype to address the toolbar bugs.