Personally I only use a handful even though there are thousands available but I get asked a lot how I can find faults in HTML so quickly, debugging JavaScript issues or finding out why a page takes so long to load. So here are my top 5 add-ons.

1. X-Ray: a fantastic tool that shows you very quickly what classes, ids and layers are in use on a given page. This is really helpful when putting a site together or debugging why it doesn’t look quite right.
2. Web Developer: now I wont go into too much detail on this one because it offers quite a lot of functionality so here is a quick run-down of what’s available: in-line CSS review, form fields view, edit HTML in loaded page, pixel ruler, outlining, quick links to w3 validation for HTML and CSS, JavaScript and CSS error reporting, disable JavaScript/Cookies/Redirects/Referrers etc.
3. ColorZilla: a nice little colour picker, always helpful!
4. IE View: this is another really basic add-on that simply adds a link when you right click on a page that asks if you want to open this URL in Internet Explorer, surprising how much time this saves!… OR do you have a site you goto that ONLY works in Internet Explorer? When you goto the site in Firefox, open Tools and select “Always view this page in IE”!
5. FireBug: this add-on lets you view, edit, debug and monitor CSS, HTML and JavaScript live in any web page.