Constant updates, add-ons and high performance across all platforms, especially Windows PCs, makes Mozilla Firefox one of the most popular web browsers on the planet. Its intuitive interface lends itself to beginners whereas the high level of customisation makes sure that advanced users are kept on their toes.
Tabbed browsing is at its heart and the add-ons is where Firefox really comes into its own by offering all kinds of ways to tinker with the browsing experience. Security is kept at an exceedingly high level thanks to the slew of updates that are always being worked on and a speed test last year showed that it is only second to Internet Explorer for speed.
Windows versions going all the way back to 95 can still use versions of Firefox yet the latest stable version (35.0.1 at the time of writing) are only available for XP Service Pack 2, Server 2003 SP1 or later versions. As for the future, stable builds are on the way all the time and for HTML5 it is just bested by Google Chrome and Opera, thus making its features exceedingly hard for any other browser to beat.