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