forgot to say my failed build was with thumb target.

thumb might work for updated gtk:
libglib2 2.24.1-1maemo1+0cssu0+thumb0
atk 1.24.0-2maemo1

problem is iirc our gtk is patched. these would need porting as well.
if focus flag exists (and the function mentioned is the only 2.18+ feature) we could just find references to it in gecko and replace with equivalent code.

Edit: mozilla prerequisites page is a bit out of date, lists gtk2.10 but configure.in requires 2.18.