Firefox can fail to spawn child processes on the first launch. After being restarted it runs fine. Consider adding more swap space for stable Firefox work.
In some scenarios Firefox hangs or crashes. In many cases enlarging the swap helps. In the example below we enlarge swap from 4 to 8 GB:
$ zfs get volsize rpool/swap
NAME PROPERTY VALUE SOURCE
rpool/swap volsize 4G local
$ pfexec zfs set volsize=8g rpool/swap
$ pfexec reboot
Desktop software and libraries