Firefox director of engineering Jonathan Nightingale posted a few thoughts that appear to be a preparation for "some adjustments" that are seemingly published to avoid Mozilla being accused of delaying Firefox releases. There is no clear information if and when such "delays" may happen, but Nightingale noted that 40 days or 44 days for release cycles make just as much sense as 42 days.
Of course, it was Mozilla that published release dates for Firefox more than one year in advance, following exactly the 6-week release cycle process for the Nightly, Aurora, Beta and Stable release versions. While this strategy almost certainly will cause media reports of a delayed release date should Mozilla miss one of its own given dates, it is unlikely that anyone would care if Firefox would be released two days earlier or later. Google, for example, has transitioned to a 6-week release cycle in fall of 2010, but never published release dates in advance. Nightingale now says that Firefox will be released when it is ready, which is much more reasonable than sticking painstakingly close to the 42-day release goal.
It may be more important to note that Mozilla recently announced that Firefox 3.6 will finally reach the end of support on April 24, with Firefox 3.6.28 being the last release for the browser. The minimum requirement for running a supported version of Firefox will be Windows XP SP2 following the discontinuance of Firefox 3.6.