Back in July 2011 Mozilla moved to the Rapid Release schedule/process for both Firefox and Thunderbird with a scheduled (does not include re-spins and security releases) new release set for every six-weeks or about 42-days. This may change in the future by a couple days in either direction. The updates will be released when they are ready whether it is 40, 42 or 44-days out.
Rapid Release was introduced for two reasons, one was to match the release cycle of Google Chrome (more on this shortly) and to ensure the massive delay in getting Firefox 4 release never happen again. The delays in getting Firefox 4 released really hurt Mozilla’s market share in 2011 (though the decline started in the fall of 2010 when Firefox 4 was originally planned for release).
While Chrome is also on a six-week rapid release cycle like Firefox, there are a couple of differences. Unlike Mozilla, Google did not post a release schedule for the entire year. Further, Chromes updates are done silently in the background so as not to interrupt the user’s browsing experience. Firefox is suppose to be moving to ‘silent updates’ soon which I expect is going to be met with dramatic mixed reviews (either love it or hate it, no middle ground).
It will be interesting to see if Mozilla does deviate from the set release schedule. They had hinted this was going to happen with the Firefox 11 release last month, because it was on the same day as Microsoft had released it monthly Windows updates. However, Mozilla did end up pushing out the Firefox 11 along with the final release on Firefox 3.6.x branch as scheduled on that Tuesday.
Source: Tom’s Hardware