Mozilla has begun offering Firefox 3.6 to users running older versions of the open source browser.

The offer was the first coordinated invitation to Firefox 3.6 that Mozilla has displayed to users of 2008's Firefox 3.0 and mid-2009's Firefox 3.5.

As with past update offers, the one for Firefox 3.6 lets users accept the upgrade, postpone the next reminder by 24 hours or decline the invitation. The latter, however, doesn't necessarily block future offers. Mozilla typically issues multiple upgrade invitations to users who have previously declined, hoping to get everyone onto the newest version.

According to Mike Beltzner, the director of Firefox, the offer screen appears after a minute of keyboard inactivity "to ensure we don't get in the way of anyone's activities."

The timing of the upgrade offer is similar to last year, when Mozilla waited about six weeks after the debut of Firefox 3.5 to pitch the new browser to users. This year, Mozilla launched Firefox 3.6 on January 21, seven weeks ago.

Mozilla has a history of convincing most users to upgrade. According to web metrics company NetApplications.com, Firefox 3.0's share of Mozilla's browser usage market plummeted from 89% in June 2009, the month Mozilla released Firefox 3.5, to under 2% in February 2010. During the same stretch, Firefox 3.5's share soared from less than 2% to 60% last month.

Firefox 3.6 already accounts for 21% of all Mozilla browsers in use, NetApplications' most recent data shows.

Elsewhere, Beltzner said that Firefox 3.0 will be retired from support at the end of this month when Mozilla ships Firefox 3.0.19, the final security update for the browser launched in June 2008. Current plans are to release Firefox 3.0.19 on March 30.