Mount the snapshot of the original file system on a temporary mount point.

# mount -t btrfs -o subvol=ext2_saved device temp_mountpoint1 

Mount the image of the original file system read-only on another temporary mount point, specifying the correct file system type (ext2, ext3, or ext4) to the -t option.

# mount -t extN -o loop,ro temp_mountpoint1/image temp_mountpoint2