Mozilla reversed its decision to block a Microsoft extension for Firefox that was disabled last week because the software was believed to represent a security risk.

The extension at issue, Microsoft's .Net Framework Assistant, has been removed from the Mozilla Add-on blocklist.

The Add-ons blocklist prevents unsafe Firefox extensions from being loaded when Firefox starts. The blocklist currently includes eight other browser extensions that present stability or security issues.

In a post on Friday, Mike Shaver, Mozilla's vice president of engineering, said that Mozilla, with the support of Microsoft, had disabled the Add-on as a security precaution.

Then on Sunday, Shaver said in a blog post that Microsoft's .NET Framework Assistant was restored following Microsoft's confirmation that the extension could not be used to exploit a serious Internet Explorer vulnerability, MS09-054.

A related plugin, Windows Presentation Foundation (WPF), remains blocked, however.

The .Net Framework Assistant provides support for Microsoft's ClickOnce software installation technology, which lets .Net apps download and run within browsers.

It's also a source of ongoing controversy because Microsoft distributed it through its Windows Update service without asking Firefox users if they wanted it. Microsoft also made the extension difficult to remove initially.

Worries about the risk posed by the .Net extension first surfaced in February. As a post on Annoyances.org put it, "This update adds to Firefox one of the most dangerous vulnerabilities present in all versions of Internet Explorer: the ability for Web sites to easily and quietly install software on your PC."

Because some Firefox users may require ClickOnce functionality, Shaver said that Mozilla is working on a way to allow users, particularly enterprise users, to override its Add-ons blocklist.

InformationWeek has published an in-depth report on new software models. Download the report here (registration required).