I love the Web Developer Extension for Firefox. I'm particularly enamored with the DOM inspector functionality (Ctrl-Shift-Y). Then you can click on an element, and a new window showing what CSS affects that element will appear. It's brilliant. Of course that's only one thing the Web Developer toolbar does. It's cookie menu renders Cookie Culler obsolete, for my needs anyway. You can also submit your pages to the W3C for validation right from the tools menu. There's several other menus that I haven't even explored yet.
I have found a few other helpful extensions:
- Colorful Tabs: This add-on will prettify your browsing by at least 25%. It's so much easier to pick out your tabs if they have different colors.
- JS View: Yet another tool for get web pages to reveal all their secrets. This extension shows external .css and .js files and allows you to view them with just one click.