There's a huge selection of addons for Firefox available.
I've found my favorites for Joomla work and recommend them to everyone.
They will save you a great deal of time, especially when trying to figure out a template is built, bug tracking and fixing and in your day-to-day work as a web developer.
Firebug
A must! Web development plugin. Inspect the site structure and experiment with the code while seeing the changes applied directly in your page. Great for bug fixing and cross-browser checking / adjustments.
FireFTP
FTP Client inside of Firefox. Very handy indeed. Although I mostly use eXtplorer to upload files, it's very handy to have this tool inside of Firefox itself.
Web Developer Toolbar
Adds a menu and a toolbar with variuos web developer tools. I use the function for highlighting DIVs all the time (creates a red line around the div the mouse hovers over). Hugely time saving!
Rank Checker
Track where your site ranks in Google, Yahoo and Microsoft Live Search using this free Firefox addon.
MeasureIt
Draw out a ruler to get the pixel width and height of any element on a webpage.
Screengrab
Saves a webpage as an image. You can save or copy either the whole page, the visible portion or a selection. Great for documentation.
IE Tab
Enables you to use the embedded IE engine within Mozilla/Firefox. Show the page as it will look in IE in a separate tab.
View Source Chart
View page source with HTML Chart GUI.
If you know of other useful Firefox plugins for Joomla users, please comment!