I just got this working in Iceweasel 38.3.0 on Jessie.
Mozilla says that 'Firefox Hello' is not available in ESR versions of Firefox but it seems that it's just disabled by default.
To enable Hello go to the 'about:config' page and set the preference called 'loop.enabled' to 'true'. 'Loop' was a previous name for the 'Hello' feature. It seems that you also need to accept cookies from mozilla.org for this feature to work.