It’s happened. My favorite Firefox plugin, firebug, is so useful and well-designed, that developers have decided to add on to it’s functionality through plugins. While these are still installed and managed the same as a normal Firefox plugin, they require firebug to be installed first. In fact, they are not extending the functionality of Firefox, they are extending firebug itself. They are plugins for a plugin.
It started (AFAIK) with Yslow. This plugin, developed by Yahoo!, analyzes the page, and determines why it is loading slowly. It is based on their own best practices for high performance websites. This is most likely not the type of tool you will use daily, but when you get to the optimization stage of a project it is very handy.
Just the other day, I had the need to analyze the cookies one of my sites was using, and to do so installed Firecookie. Overall I was impressed. It provides a dead-simple way to view, edit, and delete any cookies associated with a page. The one feature I wish it had, was the ability to temporarily disable cookies, without deleting them. This is handy if you want to quickly check what something looks like while logged out of a site, without having to actually log out, and then back in.
There are other firebug plugins I have yet to use, and I’m sure more are currently being developed. All of this added functionality strengthens my belief that firebug is becoming less of a nicety for developers, and more of a necessity. It is definitely one of the larger tools in my development toolbox.