You can find information on releases and notes for previous releases here: https://wiki.mozilla.org/Releases

I believe that the version number in front of the decimal is a major version. With new major versions, new features are added. With new minor versions, security issues are dealt with if they arise in a major version.

This link just raised more questions. Is ESR (extended support release) the designation for a major release? The next reply said the number in front of the decimal is the major release number - that can't be since that would mean that every 'whole number' release is a major one and I know that's not true. If it were true, there would be 12 major releases in the past 12 months (spanning from ver 10.0 through 19.0).

The ESR is a Extended Support Release channel (meant more for Enterprise users in mind) where it is currently 17.0.3esr (released when 19.0 was) and will only get allowed security and stability updates until oh some twelve weeks after 24.0 release where a new ESR version will be based on 24.0 release as 24.0esr.

X.0.*

X is the Major release while the * are the chemspill updates if it was deemed important (Usually for security or a crash/regression issue for example) and cannot wait for the next release. These Major versions are released without intending to have updates as for example the Firefox 11.0 and 12.0 releases never had updates.

Firefox 10.0 was released on January 31, 2012 and Firefox 19.0 was released on February 19, 2013. The releases are about six weeks except for the short delay of 18.0 due to holidays.

http://www.mozilla.org/en-US/firefox/releases/