Firefox tabs run as separate processes. However, when I close the tab in my browser, the process does not die on its own. I have to kill him manually. Is this a bug? I’m using XFCE4.16 and kernel 5.13.
Usually the Web Content processes will be the same as the number of your open tabs +1. The processes don’t immediately end when a tab is closed to make it faster to reopen a recently closed tab.
Instead of manually killing processes, reduce the Content process limit.
I typed “about:config” and went to dom.ipc.keepProcessesAlive.privilegedabout. Default value is 1 and I changed it to 0. Now, Firefox behaves as I expect it.
No it won’t.
The number Web Content processes is fixed and configurable in the settings. Content processing for all tabs is split between those processes. You may have additional processes for Web Extensions.
https://wiki.mozilla.org/Electrolysis
Within the content process limit, of course. 
You never told us what you expect. Changing that setting could actually decrease performance. From the Firefox source code:
// Keep a single privileged about process alive for performance reasons.
// e.g. we do not want to throw content processes out every time we navigate
// away from about:newtab.
pref("dom.ipc.keepProcessesAlive.privilegedabout", 1);
My bad, you are right.
I just want the browser to use as little RAM as possible.