Introduction
Firefox allows you to install Add-Ons, and the one I have found most useful for HTML coders is the Web Developer toolbar.
Instructions
To install and add-on, you first need to find it. In Firefox, select Tools > Add-ons
and then Browse All Add-ons
Search for Web Developer. Once you find it, click the Add to Firefox button.
Follow the instructions to install it.
Once it is installed, you will see the toolbar at the top of your Firefox window:
To validate your web page, bring it up in Firefox and choose Tools > Validate HTML.