I wrote a Firefox extension yesterday. Nothing very exciting--it just toggles a user preference. It's called DNSUnpinning, and can disable/enable IP address caching in Firefox. This has consequences for the same-origin policy in Web browsers: some phishing-related attacks take advantage of the fact that browsers tend to cache IP addresses for 60 seconds.
I created a developer account at the Firefox Add-ons site according to the MozillaZine page about sharing extensions. If you'd like to check out the extension, it's currently available on my DNSUnpinning page. The extensions now goes into a review process, and if it's accepted, it'll start showing up on the list of official Firefox extensions.