http://www.mozilla.org/projects/firefox/
See Current Releases.

https://wiki.mozilla.org/Releases
3.6.14 and 3.5.17 tentatively scheduled for Feb 8th
https://wiki.mozilla.org/Firefox/4/Beta

No exact rule for when Firefox stops supporting an old version. The way it works is that Mozilla supports two release versions while developing the "next" new version, and for a time (usually 3 to 6 months) supports all 3 releases, immediately after the "next" version is released. Currently 3.5 is the oldest version still being supported with security updates, with 3.6 being the current release, while 4.0 is in the last stages of beta testing and will probably be released in mid to late February (after yet another beta release and an RC version or two).

The whole schedule is fluid (stuff happens), depending upon delays with releasing the new version. Depending upon how far back you look, Firefox 4.0 is already at least 3 months behind being released (mid-November 2010 originally), based upon the "schedule" posted when Firefox 3.6 was released in mid-January 2010. Based upon that, support for Firefox 3.5 would have been dropped this month; but that too has been postponed.

Excellent! This is exactly what I was looking for! Thank you very much!

Has this policy changed after Mozilla adopted the faster release cycle for Firefox?

Yes, it has been changed a bit since this thread was started 9 months ago. Firefox 3.6.x support continues and the 4+ versions lose support when the next new version is released. Support for Firefox 7 ended when Firefox 8 was released yesterday 11-08.

Support for 3.6.x will probably end early next year when the proposed, new ESR (Extended Support Release) comes about for institutional / corporate mass installations. ESR is proposed for a 42 week version support cycle vs. the "standard" 6 week rapid release schedule.