Here's my first list of favourite addons I use with Firefox which make web development easier and quicker.
Take full or partial screenshots with ease.
An invaluable web development tool that you would be crazy not to use. It's the one tool that makes debugging and testing a breeze.
A best practiced tool to help ensure your website is doing what it needs to do be fast.
It's not easy trying to find elusive color, using Firepicker you can open up a colour picker and pick the colour within Firebug.
Whilst I don't count it as a true favourite (due to some usability quirks), Web Development toolbar is a veteran of the web developer toolbag.
Still in early development, CSS Usage can tell you what CSS selectors are in use and not in use and you can also export the clean css.
A greta plugin for those whose create wireframes for their websites. Load up the wireframe as an image into Pixel Perfect, line it up as best as possible over the current tab and you can then see just how close you are web design nirvana.
If you have specific code for specific browsers then this is a easy but not foolproof way of testing the sites behaviour and layout. Also has otherwise uses in bypassing certain site checks.
So there you have it. Anything obvious I'm missing? Anything lesser known that needs a shout out? Leave a comment below with your thoughts.