Add the Firefox browser to the list of Skype blockers that has included at various times the iTunes store, the cellular networks, and a number of entire countries from China to the United Arab Emirates. Firefox can’t prevent people from using Skype, but it has disabled the Skype plug-in that highlights phone numbers on web pages and installs links to Skype to facilitate one-click calling.

Firefox had to do this because the Skype plug-in was severely degrading its performance and causing 33,000 Firefox crashes per week:

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.

One effect cited by Firefox people is Skype’s making DOM manipulation 300 times slower. DOM is the acronym for Document Object Model, or the way that JavaScript sees web pages. (Personal note: I switched from Firefox to Chrome a month ago because I had the Skype plug-in installed and couldn’t handle the slowdown, but now I’m back.)

One of the most disturbing parts of the story concern’s Skype’s lack of response to the Firefox engineers who were trying to fix the problem:

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.

This is the first time that Firefox has unilaterally disabled a plug-in, so the move is pretty controversial among the live-and-let-live Firefox user community. Let’s hope they manage to get Skype’s attention.