I have now taken the Firefox 24.8.1 ESR source code, copied over the /debian folder from microb-engine, disabled all the patches (since they obviously wont apply to FF24 cleanly) and tried to build. Doesn't get very far before complaining about a syntax error on line 80 of python/mozbuild/mozbuild/controller/clobber.py
with open(self.src_clobber, 'rt') as fh:

Turns out we need Python 2.7 (all the other dependencies listed on the "linux prerequisites" page that matches Firefox 24 seem to be correct/new enough/whatever, its just Python that is out of date.
But it looks like http://talk.maemo.org/showthread.php?t=91341 has instructions for getting Python 2.7 going so we should be good.