Mozilla Firefox 4 Firefox 4.0 on Windows 7. Original author(s) Mozilla Corporation Developer(s) Mozilla Corporation Mozilla Foundation Initial release March 22, 2011 (2011-03-22)[1] Final release 4.0.1 / April 28, 2011; 11 years ago (2011-04-28) Repository hg.mozilla.org/mozilla-central/ Written in C++, JavaScript,[2] CSS,[3][4] XUL, XBL Engine Gecko Operating system Cross-platform Size 11.9 MB – Windows 26.8 MB – Mac OS X 13.2 MB – Linux Available in 81 languages Type Web browser FTP client License MPL/GNU GPL/GNU LGPL[5] Website firefox.com