detects microformats on webpages and exports to ical/vcard
another ff automation tool
real time social browsing - With the SamePlace extension for Firefox, you can enter a virtual place associated with each web page you visit, and meet others who did the same. realised with jabber
FireBug is a new tool for Firefox that aids with debugging Javascript, DHTML, and Ajax. It is like a combination of the Javascript Console, DOM Inspector, and a command line Javascript interpreter.
Chickenfoot for Firefox: Rewrite the Web