I believe I've fixed this.
In order to get OSS support you need you replace the file media/libcubeb/src/cubeb_alsa.c with this file http://kawais.mooo.com/cubeb_alsa.c
I havn't made a patch out of it becouse this is actually a rewrite of the original file from scratch.
I think it should not be hard to make the ALSA/OSS choice optional. Unfortunately, I lack any knowledge of how autotools work.
So if anyone knows what should be done and is interested in a proper firefox support, please help me.
Patch for 31.0 : http://kawais.mooo.com/repos/firefox-patches/oss-31.0.patch
Patch for trunk: http://kawais.mooo.com/repos/firefox-patches/oss-trunk.patch
use the --enable-oss option to configure for OSS