Web Developer:
https://addons.mozilla.org/en-US/firefox/addon/60
With it you can validade CSS, HTML(even if running at localhost), clear cookies, disable javascript, view page info like color's used, page size, and lots of other usefull things
MeasureIt:
https://addons.mozilla.org/en-US/firefox/addon/539
With it you can check the size of things on the page very easy, you can check the size of a button or anything.
Colorzilla:
https://addons.mozilla.org/en-US/firefox/addon/271
With that you can get color of things on the site, be it from a css or image, you can change red/green/blue of the color(double left click colorzilla icon at bottom left) to find a good color, without using paint or anything.
You can also get the "#rgb" or "rgb(r, g, b)" of the color by right clicking the icon.
Firebug:
https://addons.mozilla.org/en-US/firefox/addon/1843
With that you can view page source code and live change it, you can check what CSS classes get for each tag, disable some property or add/change another and some other things.