UPDATE The seems to be working:
watch -n 1 "ps aux | grep firefox && free -m"
Press F5 in Tab and watch
In Firefox, about:performance is right.
But check also this in bash if i close or open and load a page tab:
$ pidof firefox
$ pidof firefox-esr
$ pgrep firefox get the main/parentid of one or "n" profiles
$ pgrep firefox-esr get the main/parentid of one or "n" profiles
Get parentid and childid
$ ps aux | grep firefox
$ ps aux | grep firefox | grep tab
You see the parentBuildID and something of the child/tab
It looks like that you can't kill the child/tab only the parentBuildID.
ps -ef | grep firefox | wc -l