What is the best development "add on" for browsers [duplicate]

Question

I know FireBug with YSlow has to be by far my personal favourite. I also use IE developer toolbar when forced to test pages in IE. Is there any other jewels out there hiding??

Answer 1

I use the Web Developer Extension for Firefox, has tons of useful options.

_{(source: mozilla.org)}

Also like the Live HTTP Headers Extension.

Answer 2

Well you already mentioned FireBug, so the next web-related tool I use a lot would be IE Tester (http://www.my-debugbar.com/wiki/IETester/HomePage).

It's a single browser that lets you use IE 5.5, 6, 7 and 8. It's not an "add-on", strictly speaking, but I think it still qualifies :)