Mozilla will officially retire Firefox 3.6 in May, by automatically upgrading users to Firefox 12. In a similar move last year, Mozilla put down Firefox 3.5, automatically upgrading users to version 4.0. Mozilla has not set a clear date yet, but the auto-upgrade of Firefox 3.6 to Firefox 12 will occur in early May, said Alex Keybl, Firefox' release manager.

Mozilla has been considering this move for several weeks now. As IT World points out, Keybl brought up the upgrade on a Mozilla planning discussion thread in late March, explaining that the proposed upgrade was necessary for keeping users safe while browsing. Mozilla issued the last security update for Firefox 3.6 at the end of January, and on April 24 it officially retired the browser from support.

Over the last months, Firefox 3.6 users have been warned several times that they need to upgrade because of the upcoming retirement. The last message came as the final warning before Mozilla switches on automatic upgrading.

Alternative: Firefox ESR

Users who do not want to upgrade to Firefox 12 can choose to update to Firefox Extended Support Release (ESR), a new release Mozilla came up with to soothe customers, especially IT admins who do not, or cannot upgrade every six weeks when Mozilla rolls out new updates. The current version of ESR is based on Firefox 10 and receives security updates for one year. Except for the patches, it will not change until November 2012, and will be supported with updates until early February 2013.

Meanwhile, users who want to hold on to Firefox 3.6 must disable updates before the automatic upgrade due in May. To disable updates, select "Options" from the Firefox menu - or "Preferences" if you are using a Mac - then "Options" again, "Updates" and "Never check for updates."

"Users will be automatically updated unless they have specifically disabled updates," Keybl told IT World. "However, we strongly advise our users to upgrade from Firefox 3.6, as they will no longer receive critical security updates."

Last month, version 3.6 still accounted for a significant share of Firefox usage. According to metrics company Net Applications, about 13 percent of all copies of Firefox, i.e. about one in eight, were Firefox 3.6.

(reported by Alexandra Burlacu, edited by Dave Clark)