More or less related to bug 41147 (T43147). Iceweasel team may consider Mozilla's ESR version in their forking work. However that bug was turned into talk about Firefox forks, so I start a new bug here.
Firefox is said to have an "Extended Support Release" which is recommended to be used "by enterprises, public institutions, universities and other organizations that centrally manage their Firefox deployments" by Mozilla. We may want to support that version too (just an older Firefox with security bugs fixed AFAIS).