After some thoughts I would say option 2 is the best way forward (get FF24 Gecko code running in microb-engine).
To pull it off we need to
A.Identify which of the Maemo local patches need to be forward ported to FF24 and port them
B.Identify what needs to change in the debian packaging to make FF24 work
C.Identify any breaking changes to API or ABI between microb-engine and FF24 and find a solution to those somehow
D.Identify anything important that got removed between microb-engine and FF24 (such as the above linked bugs) and find a solution.
and E.Identify any cases where FF24 needs newer versions of libraries than microb-engine and find a solution to that somehow.