You turned what I said on its head. Nobody unskilled is doing this, and the buttons are there exactly because most people want them there and know nothing else. Including me, that knows about the shortcuts, prefer going caveman and click buttons on a screen. You are overestimating the base tech literacy of most humans, as most people have absolutely zero idea how to reach the settings page.

Try this one instead for the X button, before the audio button rule:

.tabbrowser-tab  .tab-close-button { display:none !important; }

And they are back despite the fix

They're intermittently there because the script is a patch but the underlying functionality has not been disabled.
Sometimes the page will override the script patch, and put unwanted buttons of my tabs,
so I end up closing and muting tabs by accident

EDIT: Also doesn't work because when a tab is open, userchrome.css does not apply until you actually click on the tab
result

I wish software was more respectful of users' abilities instead of treating everyone has grandma's tech literacy level and make decisions for them!

allow me to introduce you to my co-workers that manage the state power grid, who sit in front of a computer for eight hours a day, never knowing what a keyboard shortcut is, or that pressing 'shift+tab' can move your text cursor to the previous field, or, indeed, that the mouse wheel is also a button...