See below browser market share details provided by different companies.
60% : Internet Explorer based browsers
33% : Gecko based browsers like Mozilla, Firefox and Netscape
July – September 2005
Internet Explorer : 85.09%
FireFox / Mozilla : 12.2%
Internet Explorer 5,6 : 75.5%
FireFox / Mozilla : 20.5%
See the difference in the values as measured by different companies. For my designs, I consider the average as 85% of Internet Explorer and 15% of Firefox / Mozilla.