How do I kill a Firefox process thrashing my computer? 18.04

In short: a Firefox process takes control of Ubuntu 18.04 LTS in my Acer TravelMate P, and it seems there's nothing I can do but to shutdown the computer. This is similar to https://superuser.com/questions/248707/how-do-i-quickly-stop-a-process-that-is-causing-thrashing-due-to-excess-memory however, the main answer didn't work for me (some combination with SysRq), and I prefer not to have a script running like @tobixen suggests there.

Ctrl-Alt-T opens a terminal emulation window.

ps -ef | grep firefox shows the Firefox processes, or top will show processes, memory use, and CPU use.

kill -9 nnnnn where nnnnn is the PID number of a suspect process stops that process. You can specify multiple PIDs in one command line.

Browsers are susceptible to running Javascript of dubious goodness when you stumble upon a malicious website. Often, the website's creator is time sharing your PC to run mine Bitcoin or such.

Extensions like NoMiner block mining specifically, and other extensions like NoScript go further and block Javascript except when you let it run. Javascript is used so widely, for good and bad, that NoScript and other apps will allow you to open up Javascript for specific websites as needed, and will remember when you opened the door for a particular site.