Mozilla has barred a Skype extension for Firefox, accusing it of causing 40,000 browser crashes a week and of dramatically slowing page-load times.

"We believe that both of these items constitute a major, user-facing issue, and meet our established criteria for blocklisting an add-on," Mozilla said in a blog post yesterday. Because the extension is installed by default when Skype's main software is installed, a "large number of Firefox users who have installed Skype have also installed the Skype Toolbar, knowingly or unknowingly," Mozilla said.

Mozilla is in contact with Skype programmers and will restore the extension's privileges if the problems are addressed, the organization said.

In a statement, Skype said it's resolving the problem.

"Based on our initial investigation, we know that downloading the new client will fix for most users any compatibility issues, and are working with Mozilla to ensure that there are no other compatibility issues. We are sorry for any inconvenience this has caused our users," the company said.

The Skype toolbar extension, bundled with the Skype software for making audio and video calls over the Internet, highlights phone numbers in Web pages to make it easier to call them with Skype. Those who really like it can still run the toolbar, Mozilla said: "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 extension has been the No. 1 or No. 2 cause of crashes for the current stable version of Firefox, according to comments in Mozilla's bug tracker. And the plug-in dramatically slows Firefox's processing of Web page elements through what's called the Document Object Model (DOM)--by a factor of 3 to 8 with a newer 5.x version and by a factor of 325 with the older 4.x version, Mozilla programmer Boris Zbarsky said. The effect of this is to make pages appear to load much more slowly.

Earlier in January, a Skype representative acknowledged that the company knows about the issue. "Look out for an update in the near future," the representative said.

Updated 6:57 a.m. PT with Skype comment.