Some are indirectly related to web development but all of them have improved my development experience and are the main reason why I still do most of my development on firefox. Most of these plugins are pretty well known but I seem to constantly find interesting new plugins that are super helpful.
This is another invaluable plugin, it's awesome to be able to see what requests are sent to the server and what responses are received at the browser. Super handy for debugging redirect issues, things like http splitting attacks and just generally seeing whats going on under the hood. There is a similar tool - webscarab, which is similar but more flexible giving you the ability to edit request/response data etc.
Its really handy to have a lot of nicely tagged bookmarks in your sidebar, really easy to bookmark, tag and find pages, a brilliant reference tool if you keep using it.
Although it gets a little outshone by firebug there is so much functionality in the web developer extension that you can't pass it by. If it wasn't developed I imagine it would require a small army of plugins to accomplish everything the toolbar does.
Getting rid of flash by default speeds up my web experience and reduces memory usage, especially on Ubuntu where firefox was really struggling with flash for a while there it seemed. Easy to add sites to the white list so its not a biggie adding youtube and the like. Although I like to have youtube blocked so I can open multiple tabs of videos without them all loading/playing.
Just discovered this recently - wicked tool, useful for when you close your firefox browser with sh*tloads of tabs open. When you open the browser again only one tab is loaded, the rest of the tabs are greyed out and loaded when you select them. Browser loads super fast and helps to reduce memory usage. There are plenty of options for this plugin which can be used to reduce memory usage and improve the tab browsing experience.
Aggregates your RSS feeds from google reader and creates your own little personalised magazine, quite useful for keeping up with the latest news and salacious gossip from the web development and internet marketing communities. Also suggests awesome articles so it can be useful for a lunch break when you're not browsing reddit.
A nice and simple little time saver for measuring the size in pixels of rendered content on the browser. Nice, simple, time saver.
Another nice, simple, time saver - eye dropper tool for the browser. Especially useful is the "resample last location" tool so you can find the color of those hyperlinks in non active state.
Similar to the Net Tab of firebug but presented in a user friendly summary to help your improve the loading speed and size of your web pages which is an important factor in usability and search engine optimisation. Helped me identify areas on this blog to improve, which files needed to be gzipped, which external requests could be lumped into one. Sidenote: W3 Total Cache is awesome for this.
Search status is just useful and quick and unobtrusive. A very handy little tool for quick and dirty stats about the search engine optimization of a web page.