UPDATE The seems to be working for the usage in bash:
watch -n 1 "ps aux | grep [f]irefox"
watch -n 0.1 "ps aux | grep [f]irefox"
watch -n 1 "ps aux | grep [f]irefox && free -m"
watch -n .0.1 "ps aux | grep /usr/lib/firefox-esr/firefox"
watch -n .0.1 "ps aux | grep /usr/lib/firefox-esr/firefox && free -m"
watch -d -n 0.1 "ps aux | grep [f]irefox"
Press F5 in Tab and watch
When i kill the a tab with the right pid i get the message in the tab, that the tab is crashed now ^^
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 [f]irefox
$ ps aux | grep [f]irefox | 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, if you kill the child/tab the tab will crash but not close.
ps -ef | grep [f]irefox | wc -l