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
$ pgrep firefox-esr
$ 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.