firefox hang with virtfs

Firefox hangs once it starts to load pages. I tried to delete .cache/mozilla/ and .mozilla/ but it doesn't help. But if I mount tmpfs on to .mozilla (not necessary for .cache/mozilla/), pages loads fine.

I started the vm as root (sudo) with the following command: qemu-system-x86_64 -enable-kvm -m 4G -virtfs local,mount_tag=qemu,security_model=passthrough,path=/mnt/qemu/ -kernel /mnt/qemu/boot/vmlinuz-linux -initrd /mnt/qemu/boot/initramfs-linux-fallback.img -append 'rw root=qemu fstype=9p' -usbdevice tablet -vga qxl -spice port=12345,disable-ticketing

/mnt/qemu is a btrfs snapshot of the subvolume used as the host root

Arch Linux, qemu 2.3.0, firefox 38.0.1