When developing web pages is essential having a good set of firefox extensions. It gives you powerful tools for checking accessibility, and for getting more information on your website, and are also good fot getting information about all websites...
Those are my favourites:
Html Validator: Quick view if you are following standards.
Live PageRank: Quick view on site's PageRank.
Web Developer: Many tools, specially for disabling css or javascript, and for resizing screen.