Browser plug ins are invaluable tools for QA departments dealing with web software. Here are some very popular ones (and my favorites)…
I love this amazing, free testing tool. The Web Developer toolbar makes validations a breeze. You can perform CSS, HTML and link validations right from your menu bar.
It allows you to outline different page elements and display their title- which is really helpful when creating Selenium test scripts. You can also view your page without cookies or at a different resolution. This is one of the most helpful tools out there and I won’t sign off on a release with out it.
Yslow is awesome. It works within the firebug window. Click over to the Yslow tab, open up the menu, and it grades the page you are on for performance. It even offers suggestions on how to make it faster.