Firefox 3.0 users have until January 2010 to switch to the newer Firefox 3.5. At that time, Mozilla will halt production of security patches for the older version.

Last year, for Firefox 3.0 about four weeks after it launched the browser. The delay this time around -- the offer for Firefox 3.5 follows that edition's debut by about six weeks -- was not explained by Mozilla.

Mozilla's eagerness to get users onto the newest Firefox is in stark contrast to Microsoft's policy with Internet Explorer (IE). Even though grassroots campaigns that urge users to dump IE6 are gaining momentum, Microsoft said last week it won't . Several managers in the IE group have cited enterprise reliance on IE6 as the reason why it's not feasible for it to push people to a newer model, such as this year's IE8.

from Web measurement company Net Applications, about 20% of all Firefox users are already running 3.5, while slightly more than 72% are still using Firefox 3.0. The remaining 8% are running versions older than Firefox 3.0.