Running firefox inside firejail on xephyr with firejail --x11=xephyr --noprofile firefox works, however 2d performance is bad on some html5 games. On unjailed xephyr i can start firefox with vglrun firefox, however doing that in firejail gives me a black screen.
So you get only a black screen when you run firejail --x11=xephyr --noprofile /usr/bin/vglrun /usr/binfirefox and xephyr is firejailed too (via firecfg symink).
Running firefox inside firejail on xephyr with firejail --x11=xephyr --noprofile firefox works, however 2d performance is bad on some html5 games. On unjailed xephyr i can start firefox with vglrun firefox, however doing that in firejail gives me a black screen.