Version 12 is not scheduled for a final release until April 24 of 2012. According to the recent platform meeting minutes, the removal of the OS security dialog in Windows was moved out to version 11 and background updates were moved to version 12. "Risk" was the reason for both delays.
While silent updates are a somewhat controversial feature that could alienate some Firefox users who want to retain control over the browser by default, there is no doubt that the mainstream user actually prefers silent updates. Chrome has benefited from silent updates since its launch in 2008 and has since then changed the perception of silent updates being "forced" updates to "convenient" updates.
The delay on Mozilla's side raises concerns about the overall competitiveness of the browser, given a recent development that saw Firefox' market share dropping from second to third rank - behind Google's Chrome. Mozilla can't afford any delays in its product roadmap and this one could have impact on Firefox' market share in 2012 as well as chances at a successful comeback.
The history of browser market share has shown that new features in browsers typically require a setting time of about four to six months to show effect. Other new features waiting in the pipeline include Chrome migration tools and the Home Tab App.