Useful Firefox Extensions

One of the better reasons for using Firefox are it's extensions. Currently I am using the following:

• Web Developer Very handy toolbar that gives a lot of essential functions for anyone who does ANY work with html.
• HostIP.info Pointless really unless you are like useless info. Has already turned up some interesting facts :)
• User Agent Switcher Another good extension which is good at turning up interesting ways of getting into sites!
• Flashblock I tend to dislike useless flash ads, so anything that kills them while leaving the option of clicking on flash games for a bit of distraction is always going to be handy.
• ShowIp With the work I do on a day to day basis, it is very handy to know what IP address you're looking at when you view a site. This simple extension displays the IP of the site in the status bar and offers shortcuts to online DNS utilities if you click on the IP.
• Switch Proxy My laptop is thrown onto lots of different networks, often with different proxies. This extension gives me a nice drop down menu with a list of proxies and can be a nice time saver.

Is there any other handy extensions floating around the net? I don't want to totally bog down Firefox, but things that make life easier are always welcome.