Zramup script:

    #!/bin/sh
    doas /sbin/modprobe zram
    doas /sbin/zramctl --find --size 1024M
    doas /sbin/mkswap /dev/zram0
    doas /sbin/swapon /dev/zram0 --priority -1

Not Firefox, but Luakit might do it fine on single pages where JS is mandatory, such as, sadly, my country's goverment pages to do personal life related tasks.