With so many extensions out there and so little time to choose them I thought I would share the ones I have installed and use all the time.
Simply a must have for any web developer, allows you to inspect the webpage, its elements, css, script, downloads size and speed, edit css, script and html in line. My favourite bit is helping debug ajax issues. Get FireBug FireBug also has an architecture that aloows plugins. You can get your ColdFusion debugging to display only in FireBugwith the ColdFire Extension. There is also the same treatment for PHP with the FirePHP extension.
Web Developer Toolbar
I love this little tool for its simplicity, it simply gives you a colour picker for your webpage. You need to know the hex or RGB values of a clour on screen then ColorZilla is for you.
I dont like having Internet Explorer open at the best of times, but unfortunately my job dictates I must have websites working in IE. This plugin puts a button on my status bar allowing me to switch rendering engines within FireFox to and from Internet Explorer. IETab
A simple plugin that allows you to draw a ruler or a container over the top of everything else and find out how big it is. Makes it much easier than having to trawl through the code to find a css width and the like. Measure IT
Net Usage Item
The Home page doesnt do it justice, but this plugin will sit in your browser and tell you your usage stats for about 35 Australian ISPs. It also has ISPs in New Zealand, India, Canada, Belgium, and Ukraine. Net Usage Item for FireFox
I am a massive tab fan, I love it how you dont clutter up the Windows taskbar with more than one instance of FireFox. However I get so many windows open that distinguishing between them can be hard. Chroma Tabs takes the URL of the site and converts it to a colour, then applies it to that tab.
Very simply generates paragraphs of Lorum Ipsum text for sample pages and layout testing. Has a right click menu when you are in a form textarea to insert directly with no copy paste. Dummy Lipsum generator
A Download manager extension that speeds up downloads, allows restarts, multiple files and multiple streams. Put simply this is a fantastic piece of software to manage your downloads better, and get things down faster. Love it.DownthemAll
FoxMarks allows you to sign up to an account and have your bookmarks synchronised to their server. When you rebuild your machine or start a new job, install foxmarks, sign in and your bookmarks get synched. Its a del.icio.us style bookmark synchroniser, but I like it better and its not public. FoxMarks Bookmark Synchroniser