Here is an overview of add-ons I consider essential to any web developer or designer who uses Firefox.
Have you ever seen a really nice colour on a website and thought “That is exactly what I need”. This add-on allows you to get the details about the colour.

Websites can be very annoying to code (especially if you use notepad), this excellent add-on for Firefox allows you to edit, debug and monitor code on a web page. It’s so useful it almost needs its own post.
YSlow (For Firebug)
Ever wondered why a website is being slow? This cute little add-on (made by Yahoo) analyses a web page and tells you (in simple English) how to make it load a little faster.

Normally when I use this, I notice a big improvement in rankings on Google and Yahoo.
Sometimes (especially when coding websites) it’s important to know the size of various elements. Essentially this is like an on-screen ruler.

An essential add-on for all web masters (and power users). This useful little tool bar adds everything a web master really needs to keep on top of a website.
I hate JavaScript; luckily the geniuses down at Google seem to know how I feel. Google Gears makes time consuming JavaScript run at least 40%* faster.
View and Modify HTTP/HTTPS headers, very useful when analysing a web page and working with CURL.

* Based on my benchmarking. Performance may vary from user to user. In general Google Gears is fantastic and should be used on all computers.
There is nothing wrong with brining the Taleban on the program, but I, too, wonder how forth coming they are and how the actual interview was conducted. ,