Web Developer toolbar
For Firefox fans
the Web Developer toolbar provides a handful of useful tools for developing and troubleshooting web pages. View, edit and modify CSS styles while watching your page update on the fly. Use the outline tool to show the structure of divs and block level elements. Validate pages with any number of web standards and compliance measures. Ruler, magnifier, guides, cookie management are only a few of the many tools provided in this essential development tool.
Microsoft also supplies a web developer toolbar for IE that you can find here.

March 23rd, 2007 at 11:22 am
I can attest that this extension is one of the better ones out there. Not as good as Firebug, but pretty close.