After trying to comb through the files changed between esr38 and tb_GECKO380esr_2015050513_RELBRANCH (tb release), it's clear that it is not going to be easy or direct to start building on top of the tb release branch.
The apk built on the tb release branch seems to fail and is not easy to debug, so, I have decided to manually apply patches between those two branches and test it out hoping to find the code that is causing this failure, although this might not be the best approach, it seems to be the fastest way of finding the actual problem.
The esr38 branch, luckily builds and launches just fine and so, working from that point might provide good results.
Patches 0001-0039 produce an apk that works fine but, when 0040-0046 are applied, there arises a problem. I've finally managed to boil it down to that much, and that might actually be the only problem as the apk I currently have behaves exactly like the one built on the tb release branch. I will look through this and hopefully have a working patch tomorrow.