There will no longer be security updates for version 4 of Firefox.
That was a quiet detail in all the information surrounding Mozilla's recent release of Firefox 5, which doesn't add a whole lot of features to the browser but makes the market for add-ons much more flexible.
Users who have gotten used to Firefox 4 should find the update to version 5 pretty unobtrusive. The radical interface and feature changes that highlighted changes between Firefox and Firefox 2, Firefox 2 and Firefox 3, etc, are not an issue here.
In fact, Firefox release manager Christian Legnitto said, "Firefox 5 will be the security update for Firefox 4," suggesting the adjustments between versions were largely behind-the-scenes and not cosmetic.
The launch of Firefox 5 happened this week, catching some by surprise since Firefox 4's release was itself pretty recent.
Users should have received a notice stating, "A security and stability update for Firefox is available. It is strongly suggested that you apply this update for Firefox as soon as possible." So users would have to specifically opt out of the update to remain with Firefox 4 and, thus, remove themselves from future security updates.
Before Firefox 5, only one update had been rolled out to Firefox 4 - Firefox 4.0.1.