One of the best features of Firefox is that it can easily be improved by using extensions. Talked about it before in February - Firefox Extensions (for Firefox 0.8). No developer extensions (that I can find) since then (new features added to existing ones, updated due to version changes, added to Mozilla Update), but that may simply be because the current ones do what most developers want. Here are the useful ones to web developers:
- Web Developer Extension, by Chris Pederick, the best one so far
- IE View - for those rare times when you need to open a page in Internet Explorer, or for testing purposes
- LiveHTTPHeaders - for viewing HTTP Headers you send/receive to/from websites
- EditCSS - editing CSS live on a website in your side bar (you can do this with the Web Developer Extension as well).
- User Agent Switcher, also by Chris Pederick. To make some sites think you are using a different browser.
- Web Panel Enhancer - allows you to do various things in the sidebar, like open link, current page, or show source code
There is also a tutorial available on creating extensions for Firefox.