Highly recommend the Firefox addon Firebug. Been using it for a couple of days and wondering how I managed without it. The element inspector is great - just hover over an item on your site and see exactly why it's not doing what you want it to!

Has now become an essential addon for me, along with:

Adblock Plus (Gets rid of just about every advert you can think of.)
Echofon (Twitter addon, linked to your account.)
Delicious (Handy buttons for tagging pages to Delicious.)
English Dictionary (Underlines and provides options for spelling mistakes.)