There's the "dom.event.clipboardevents.enabled" preference, which you can set to false in about:config.

I don't know whether that's planned to stick around or whether it was added when the clipboard event support was still experimental and will be removed at some point. But I suspect the former, for precisely the reasons in this thread.

Note that disabling these events will likely break "smart" copy/paste in things like Google Sheets, which is one reason it's not disabled by default....