Chickenfoot for Firefox

Chickenfoot is a Firefox extension that puts a programming environment in the browser's sidebar so you can write scripts to manipulate web pages and automate web browsing. In Chickenfoot, scripts are written in a superset of Javascript that includes special functions specific to web tasks.

Firefox's About: pages and ErrorZilla

Was looking around for Firefox about pages, found out that there's quite a few interesting ones as listed in here.

Another interesting find was the ErrorZilla extension, which customizes your error page with some more tools (somewhat like a custom 404 page). [Edit] The link to install the extension is not available anymore, but you can get similar extensions from Firefox's add-on gallery.