3 indispensible Firefox Web Developer addons
I know there is a "5 applications you can't live without" thread that has spanned years - but this is specific to Firefox Web Developer addons.
I rely - heavily - on the Web Developer add on in Firefox
Click on anything on the page and view it's nested css ... handy for those obscure Drupal styles
Also useful: Measure It (how many pixels to move that?)
And color picker ColorZilla
(for some reason this addon does not appear in the site search, I had to search mozilla.org from Google)
And anything else that strikes your fancy - I have a page load timer that prompted me to sign up with a different host today - and when placeholding I grab DummyLipsum...