Web developers who utilize Firefox will most definitely want to check out the Web Developer Toolbar. This free plugin adds a toolbar along the top of Firefox to do dozens of useful things such as:
-Table highlighting
-Removal of style data
-Quick links to W3C’s validator
-Cookie overrides
-Dozens more functions too long to list here

I cannot say enough good things about this plugin as it has saved me several hours of work with its “realtime” debugging features.

Download it here: [http://chrispederick.com/work/webdeveloper/]