Install debian in a chroot using debootstrap

bind mount /proc and /dev

sudo mount -o bind /proc /path/mychroot/proc
sudo mount -o bind /dev /path/mychroot/dev


Start Xnest as display 1

Xnest -geometry 3832x2097+0+0 -ac :1 &


Start firefox from the chroot in the nested X

sudo chroot /path/mychroot firefox --display=:1