Mozilla Firefox is a free, open source web browser available for Windows, OS X and Linux, with mobile versions available for Android. It uses the Gecko rendering engine which is based on current web standards.
Features include tabbed browsing, a spell-checker, live bookmarks, Smart Bookmarks, a download manager, private browsing, location-aware browsing and integrated search. It supports extensions for adding new, third-party features, and a wide variety of these are available.
The browser also provides a web development environmenet featuring tools such as Error Console or DOM Inspector and extensions such as Firebug.