Mozilla has revealed several security features planned for Firefox 3.0, the alpha 8 preview of which has just been released.

"Gran Paradiso," the codename for Firefox 3.0, will feature built-in anti-malware warnings and protection against rogue extension updates, according to Mozilla documentation.

The malware blocker, which was first mocked up in June, will block web sites thought to contain malicious downloads. The feature, a companion to the phishing site alert system in the current Firefox 2.0, will use information provided by Google to flag potentially dangerous sites, warn users, and automatically block access to the site.

The browser also features a means to prevent plug-ins' automatic updates from sending users to malicious sites where they might be infected by attack code or drive-by downloads.

Firefox relies on small plug-ins - called "extensions" in the Mozilla vernacular - for much of its power and flexibility. Several thousand extensions have been written, the vast bulk of them by outside developers, that do everything from boost browsing speed to block irritating Flash animations. Firefox regularly checks to see if the installed extensions are up to date, and if not, automatically pulls in the newest version and installs it.

"Firefox automatically checks for updates to add-ons using a URL specified in the add-on's install manifest," Mozilla says in a developer document. "Currently there are no requirements placed on these URLs. In particular, [they are not] required to be https. This allows either the update manifest or the update package to be compromised, potentially resulting in the injection of malicious updates. A demonstration of one form of compromise is already public."

Most extensions are hosted on Mozilla's own servers - at the servers feeding its Add-ons site - but some are not; it's those off-site extensions that Mozilla wants to lock down.

To stymie attacks through a compromised extension update, Mozilla will require updates - both the actual update package and the much smaller "manifest," or notification of an update - to be delivered over an SSL-secured connection. Or the update must be digitally signed.

The change doesn't affect the initial installation of an extension, something Mozilla recognised. "[This] has no impact on the security of initial add-on installs," it told developers in the online guide.

This newest preview, which can be downloaded in versions for Windows, Mac OS X and Linux from the Mozilla site, still comes with a warning to end users. "Alpha 8 is intended for web application developers and our testing community. Current users of Mozilla Firefox should not use Gran Paradiso Alpha 8," the browser's release notes.

Mozilla has not officially committed to a release date for the final version of Firefox 3.0.