One of the primary reasons behind Firefox recently being cited as the most secure browser available was the high rate of people upgrading the browser. The in-built download-and-update function of Firefox certainly helped with that, but Mozilla has still not convinced all Firefox 2.x users to switch to Firefox 3. Many of them stay behind due to concerns about plugin compatibility, and perhaps many others simply don't know or don't want to be bothered.
Mozilla wants to encourage those people to upgrade, feeling that enough time has passed for plugins to be updated and people to be assured that 3.0 is the future. Thus, they are getting ready to trigger an update of Firefox 2 to Firefox 3 using the upgrade feature built into the browser. The update is of course avoidable, but apparently it will continue to notify the person that 3.0 is ready to be installed.
Mozilla's reasoning are numerous, but clearly supporting only a single branch of the browser will assist them in making sure that Firefox 3.0 remains a good browser.