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The first challenger to Internet Explorer's dominace of the late '90s and early 2000s, Mozilla's Firefox is a very typography friendly browser with good OpenType support.

Firefox overview

Global Market share: 23.1% (May 2012)
Font formats: Firefox supports TrueType fonts (.ttf) as well as OpenType fonts with TrueType (.ttf) or PostScript (.otf) outlines. (Since 3.6 also WOFF)
Notes: Same-origin rule: By default, Firefox will only accept relative links. If you want to use absolute links or include fonts from different domains, you need to send these fonts with Access Control Headers.

Latest version: Firefox 15

Release: September 2012
OS: Most major Windows releases, Mac OSX 10.5-current, Linux kernel 2.2.14 and newer
Global Market share: 12.49% (September 2012)
Notes: Includes support for CSS Word-Break property

Firefox 14

Release: July 2012
OS: Most major Windows releases, Mac OSX 10.5-current, Linux kernel 2.2.14 and newer
Global Market share: 3% (September 2012)

Firefox versions 5-13 (Rapid release program)

Release: June 2011 to March 2012
OS:Most major Windows releases, Mac OSX 10.5-current, Linux kernel 2.2.14 and newer
Global Market Share: 2.17% (June 2012)

Firefox 4 (no longer supported)

Release: March 2011
OS:Most major Windows releases, Mac OSX 10.5-current, Linux kernel 2.2.14 and newer
Global Market Share: Less than 1.5% (May 2012)
Notes: Introduced OpenType font-feature-settings support.

Firefox 3.6 (no longer supported)

Release: January 2010
OS:Most major Windows releases, Mac OSX 10.5-current, Linux kernel 2.2.14 and newer
Global Market Share: 2.8% (May 2012)

Firefox 3.5 (no longer supported)

Release: May 2009
OS:Most major Windows releases, Mac OSX 10.5-current, Linux kernel 2.2.14 and newer
Global Market Share:Less than 1.5% (May 2012)
Notes: Introduced support for downloadable fonts.

Stats via W3Counter