Hi Pablo,
Unfortunately, SGS2 is not well supported in mainstream project (in part, because of me). I've been maintaining this phone since the begining but right now I don't have much time, so it's a little abandoned.
If you want to try it anyway, I will suggest you to use my fork: git clone git://
github.com/atilag/B2G.git, I updated it about a month ago but my intention is to keep it updated frequently.
I have a fork of Siyah Kernel too, modifed to fit better with FirefoxOS, if you feel confident compiling kernels you can build it by yourself, just clone these two repos: git://
github.com/atilag/siyahkernel3.git, and git://
github.com/atilag/initramfs3.git, if you don't want to compile _anything_ I did an installable CWM Recovery .zip that will flash the modified Siyah Kernel and FirefoOS as well (Gonk, Gecko and Gaia). I couldn't test it so mucho so if you have any problems installing it, just tell me and I'll try to help you.
Regards,
Juan Gómez.
irc: _AtilA_