Mozilla transitioned to a "Rapid Release" cycle with the release of Firefox 4 and 5. This means that every 6 months, a new version of Firefox comes down the pipeline. https://wiki.mozilla.org/RapidRelease http://blog.lizardwrangler.com/2011/08/25/rapid-release-process/.

Basically what this means in practice for users is every 6 weeks, all new features, bugfixes, improvements, etc. that are ready in Firefox are released. If a feature isn't ready, it is held back until the next release. Instead of having to wait a year or more between updates (like 3.6 to 4.0) that contain new web technology, performance improvements, etc. New features are ready for users much more quickly.

If you wish to view the release notes, you can go to http://www.mozilla.org/en-US/firefox/fx/, click the Desktop Tab (since it will most likely be showing the mobile tab right now). Then click "What's New". This will take you to a page like http://www.mozilla.org/en-US/firefox/13.0.1/releasenotes/

Firefox 9.0 was December 20th, 2011 http://www.mozilla.org/en-US/firefox/9.0/releasenotes/

Firefox 10.0 was January 31, 2012 http://www.mozilla.org/en-US/firefox/10.0/releasenotes/

Firefox 11.0 was March 13, 2012 http://www.mozilla.org/en-US/firefox/11.0/releasenotes/

Firefox 12.0 was April 24, 2012 http://www.mozilla.org/en-US/firefox/12.0/releasenotes/

Firefox 13.0 was June 5, 2012 http://www.mozilla.org/en-US/firefox/13.0/releasenotes/

These releases are more like bite sized servings in a more controlled fashion versus the huge update every looong while of before and Firefox 4.0 was certainly a example of what not to do as it was just delayed a number of times and the amount of features and development.

I admit I might have preferred a little longer periods like say ten or twelve weeks versus six though to have a little more breathing room for the stable release part, not the development channels though.

Before the Firefox 1.0 days the releases were faster at what three or four a year.