A Couple of Simple Firefox Extensions

Server Spy is a neat little extension that let's you see the web server and version number, as well modules like PHP and GZIP that are installed on it, of the web page you are viewing. Good for administrators of multiple web servers. It makes it easy to see which servers you have where. And good for development in general.

If you find that you often navigate or explore a site by editing the url to go up one level in the path, then Uppity is a useful extension to help you do that easily.