Header Spy - nice addon for the geeky types, shows you the web server and platform a web site runs on in the status bar. It has helped me track down nefarious problems, especially when debugging issues when deploying your web app between environments. Lets you spy on all HTTP headers communicated back and forth between client and server. Live HTTP Headers - invaluable for troubleshooting, use it frequently. Although I must admit, I sparingly use this one since firebug has largely won me over. JavaScript Debugger - has a few more features than javascript debugger provided by firebug. Incredibly valuable for poking and prodding your pages, and seeing how your web app responds when used with slightly malicious intent. prior to posting back to a page, or getting a page.

Tamper Data - lets you tamper with http headers, form values, cookies, etc. I mostly use it for quickly validating pages, disabling javascript (yes I disable javascript sometimes, don't you?), viewing cookies, etc. Web Developer - again another great developer productivity tool. FireBug - a myriad of productivity enhancing tools, includes javascript debugger, DOM inspector, allows you to edit the CSS/HTML on the fly which is highly valuable for troubleshooing layout and display problems.

Firefox add ons