The timeline for Firefox releases around the end of the year has been modified and versions scheduled for that period will roll out with a delay of one week. The reason for this is not technical or development issues, but the winter holidays.
With the amount of feedback dropping significantly in that time of the year, Mozilla decided to delay the release of browser versions scheduled for that period. During the holidays of 2011, the average of incoming bug reports was of about 150 per week, instead of the regular 350.
This means that Firefox 18 will be available in the week of 01.06.2013 instead of by the end of this year. So versions 18, 19 and 20 spend an extra week in development and support for version 17 will be extended to seven weeks. Then the schedule returns to the normal 6-week release cycle.