[joveian]

That removes your middle mouse button entirely! (Unreliably due to the use of numbers with xinput, which can change; xinput can take names to avoid that issue). The issue will remain if the primary selection is pasted into a terminal by other means (personally, I use a key combination rather than mouse button) and won't be an issue if clipboard.autocopy is set to false in about:config in Firefox (per torgard's comment).

I've also set dom.event.clipboardevents.enabled to false since I don't want websites messing with the clipboard.