[detaro]

Literally right below that it says: "To address this packaging issue, once a ESR cycle is over, Debian has been accepting uploads of new ESR releases in the stable release."

As in, they accept and release new versions from upstream instead of maintaining their own fork with backports.

[Scarbutt]

But most users don't want the ESR version, they want the latest non-ESR version, yes, you can go and get it from mozilla but some users find this inconvenient.

[detaro]

Then they can get it from outside the stable branch. Testing and unstable both have a Iceweasel 44, and there is the mozilla repository at http://mozilla.debian.net/ just for mozilla software.

Debian stable is "conservative and with as few changes as possible", so the ESRs are the obvious choice for it.