The two-part Microsoft component -- an add-on dubbed ".NET Framework Assistant" and a plug-in named "Windows Presentation Foundation" -- have been blocked by Mozilla as a precautionary measure, said Mike Shaver, the company's head of engineering.

"Because of the difficulties some users have had entirely removing the add-on, and because of the severity of the risk it represents if not disabled, we contacted Microsoft today to indicate that we were looking to disable the extension and plug-in for all users via our blocklisting mechanism," Shaver said in an announcement posted Friday night to the company's .

Mozilla maintains an add-on/plug-in that automatically bars risky software from being used by Firefox. The open-source company first used the blocker in 2007. Mozilla has used the tool only nine times, including Friday's blocking of the Microsoft add-on and plug-in. In May 2008, for example, Mozilla added a for Firefox to the blocking list when the pack was found to contain a worm.

According to Shaver, Microsoft gave Mozilla the go-ahead to block the .Net Framework Assistant and the Windows Presentation Foundation.

Last week, Microsoft's security team acknowledged that its software -- which had been silently installed in Firefox as far back as February 2009 -- that could be used by hackers to hijack Windows PCs. The same vulnerability also affected all versions of Internet Explorer (IE), including the newest version, IE8.