do you have gapps? are there many addons in firefox?
don't use disk swap. it's almost never a good choice.
crashing to boot animation sounds very weird. i haven't experienced it since CM10 (android 4.1.2). perhaps there's a different issue with that rom?
i think you may have to try another rom, or a lower android version. in my experience android 12 and higher is much heavier/sluggish than 11. i'd recommend downgrading.
the android build system is really shitty. it's too optimized for OEM's high spec build servers, us poor plebs can't run it well.. it even needs insane storage space for some reason. for small modifications manual edits work much better
(and btw what device is this?)