I know service workers are used for PWAs and notifications, among other uses.

There's an internal webpage in most browsers that shows you what service workers you have installed (Firefox: "about:serviceworkers"). The answer is a lot, because every web page you visit can install one without your permission. They seem like small javascript applets, and I'm not sure under what conditions they are allowed to run. I am afraid they could be allowed to run even when you are not visiting the page, and since I have no interest in PWAs or notifications I just disabled service workers entirely. You can do that in about:config.