This took longer than it should have, but a page is now officially turned. I uploaded Firefox and Firefox ESR to Debian unstable. They will have to go through the Debian NEW queue because they are new source packages, so won’t be immediately available, but they should arrive soon enough.
People using Iceweasel from Debian unstable will be upgraded to Firefox ESR.
Debian stable will receive Firefox ESR after Iceweasel/Firefox ESR38 is end-of-lifed, in about 3 months.
Thanks go to Sylvestre Ledru, Mike Connor (the same who filed bug 354622) and Stefano Zacchiroli.