CWmike writes "Unnoticed in the Tuesday release of Firefox 5 was Mozilla's decision to retire Firefox 4, shipped just three months ago. Mozilla spelled out vulnerabilities it had patched in that edition and in 2010's Firefox 3.6, but it made no mention of any bugs fixed in Firefox 4 on Tuesday, because Firefox 4 has reached what Mozilla calls EOL, for 'end of life,' for patches. Although the move may have caught users by surprise, the decision to stop supporting Firefox 4 has been discussed within Mozilla for weeks. In a mozilla.dev.planning mailing list thread, Christian Legnitto, the Firefox release manager, put it most succinctly on May 25: 'Firefox 5 will be the security update for Firefox 4.' Problem is, users are being prompted to upgrade now but are hesitant because the new rapid release of updates means many add-ons are not compatible. And without security updates in between, many could be left exposed with unpatched browsers."