What Firefox add-ons do you use that are useful for programmers?
Some of which has been missed above are here
Load Time Analyzer – View detailed graphs of the loading time of web pages in firefox. The graphs display events like page requests, image loading times etc.
Poster – A must have tool for web developers enabling them to interact with web services and other web resources.
Aardvark – A cool extension for web developers and designers, allows them to view CSS attributes, id, class by highlighting page element individually.
Groundspeed, is useful for testing server side code. It was created for input validation tests during pentest, but can be useful for any test that require manipulating input (similar to TamperData).
It lets you control the form elements in the page, you can change their type and other attributes (size, lenght, javascript event handlers, etc). So for example you can change a hidden field or a select to a textbox and then enter any value to test the server response and stuff like that.
I guess it's silly to mention Firebug -- doubt any of us could live without it. Other than that I use the following (only listing dev-related):
I'd also recommend the Web Developer extension by Chris Pederick.
Here are mine (developer centric):
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.
Web Developer - again another great developer productivity tool. I mostly use it for quickly validating pages, disabling javascript (yes I disable javascript sometimes, don't you?), viewing cookies, etc.
Tamper Data - lets you tamper with http headers, form values, cookies, etc. prior to posting back to a page, or getting a page. Incredibly valuable for poking and prodding your pages, and seeing how your web app responds when used with slightly malicious intent.
JavaScript Debugger - has a few more features than javascript debugger provided by firebug. Although I must admit, I sparingly use this one since firebug has largely won me over.
Live HTTP Headers - invaluable for troubleshooting, use it frequently. Lets you spy on all HTTP headers communicated back and forth between client and server. It has helped me track down nefarious problems, especially when debugging issues when deploying your web app between environments.
Header Spy - nice addon for the geeky types, shows you the web server and platform a web site runs on in the status bar.
MeasureIt - I don't use this all too frequently, but I've still found it valuable from time to time.
ColorZilla - again, not something I use all that frequently, but when I need it, I need it. Valuable when you want to know a color and you don't want to dig through a CSS file, or open up a graphics editing app to get a color embedded in some image.
Add N Edit Cookies - this has been a great debugging tool in web farms where the load balancer writes a cookie, and uses the cookie value to keep your session "sticky". It allowed me to switch at will between servers to track down problems on specific machine. Also a good tool if you want to try to mess with a site that uses cookies to track your login status/account, and you want to see how your code responds to malformed or hacked info.
Yellowpipe Lynx Viewer Tool - yeah I know what your thinking, lynx, who needs it, its so 1994. But if you are developing a site that needs to take web accessibility into account (meaning accessible to users with visual impairments who use screen readers), or if you need to get a sense of how a web spider/indexer "sees" your site, this tool is invaluable. Granted, you could always just go out and grab Lynx for yourselfhere's the windows xp port that I use.
I've got a handful of other addons that I've used from time to time that I'll just quickly mention: FireFTP (one I installed wasn't stable and I've not tried a newer release), Html Validator (also found this one unstable, least back when I installed like a year ago), IE Tab (I usually just have both IE and FireFox open concurrently, but that is just me, I know many others that find this addon useful).
One that wasn't mentioned yet is this HTML Validator extension that I found very useful.