What's fundamentally new is the rapid release process. This is a new way of thinking about making software. The basic idea is that you get al lot of small updates instead of one big update every year. See:
rapid release process
rapid release follow-up

Thank you knorretje for the lead-in and links.

While this did answer my question, I certainly was not happy with the answer. Almost everyone hates this concept and since FF is prides itself on being open, stable, compatible, it seems that no one in the community really agrees with this process.

Why so many completely new numbered versions. We've gone from version 3 to 13 within a bit over a year seriously. It used to be small incremental updates like 3.10- 3.11, 3.2, 3.3 etc now It's a whole new version every single freaking month which is totally uncalled for. You've no idea how insane it drives me smh.