Mozilla Firefox

Mozilla Firefox is a user-friendly, extendable web browser. It features popup blocking, tabbed browsing, RSS integration as well as support for CSS 2.01 and XHTML. It is open source and cross-platform.

HTML and XHTML

 * The HTML 4.01 Specification is fully supported.
 * The XHTML 1.0 Specification is fully supported.
 * The XHTML 1.1 Specification is fully supported, even when correctly served as.

CSS

 * CSS 2.1 Specification is mostly supported. CSS 3.0 support has already been started, notably the opacity property.

Extensions
Extensions are small add-ons that add new functionality to Firefox. They can add anything from a toolbar button to a completely new feature.

Live HTTP Headers
The Live HTTP Headers extension adds two features to Firefox that are useful for web development. It adds a new Tool that allows one to monitor HTTP request and response headers as requests are being processed. It also adds a Headers tab to the View Page Info window with panes for the HTTP request and responses headers.

Web Developer
The Web Developer extension is invaluable for people who design and program Web sites. The extension adds a toolbar to Firefox which allows one-click access to a range of functions, including disabling images, CSS or JavaScript; labeling all the CSS "divs" in a page; outlining images without "alt" tags; and validating a page's code.

Firebug
The Firebug extension offers a huge number of tools for web designers, including real time CSS and HTML debugging.

SEO For Firefox
Want to know why a particular site ranks on Google or Yahoo!? Aaron Wall's SEO For Firefox extension takes data from a huge range of sources (including Yahoo backlink counts, DMOZ listing and Compete.com) to illustrate at a glance why a site rnaks.

Domain Lookup
The Domain Lookup for Firefox extension lets users perform one click domain lookups at DreamHost and 30+ other registrars. It uses the selected text in your Firefox window as source text.