This looks pretty cool, got potential ...
First official alpha release of Firekeeper - Intrusion Detection and Prevention System for Firefox is available for download. Firekeeper adds additional layer of protection to the browser. It uses flexible rules similar to Snort ones to describe browser based attack attempts. All incoming HTTP and HTTPS traffic is scanned with these rules. HTTPS and compressed responses are scanned after decryption/decompression. Suspicious response can trigger an alert that is displayed to the user or can be automatically cancelled, depending on an action specified in the rule. Firekeeper uses very effective pattern matching engine from Snort and with well written rules doesn't have a negative impact on the browser performance.
More information, screenshots and download links can be found at:http://firekeeper.mozdev.org
Project is GPL licensed.