Checky is a document validation and analysis extension for Firefox, Mozilla, and Netscape. It provides an easy-to-use interface to many free and commercial online validation and analysis services. It allows the user to validate, analyze, view, and inspect documents containing HTML, XHTML, CSS, RDF, RSS, XML, P3P, hyperlinks, metadata, and many more. It can check Section 508 and WAI compliance of documents with many helpful services. It can help in the design, development, and maintenance of accessible documents for a semantic web.
CookiePie is a Firefox extension enabling you to maintain different cookies storage in different tabs and windows. You can open multiple Gmail/Yahoo Mail accounts in different tabs and windows simultaneously. Developers can use CookiePie to test their Web software with different application users without the need to open different browsers.