That still works just fine, I use it all the time too. It's just when you middle-click in a text box that it pastes the clipboard. Otherwise, what will middle-click do, nothing?

… wait, I was pretty sure the behavior used to be to paste whatever you had most recently highlighted. When did that change? Or am I crazy?

That's the behaviour unless you're hovering over an active link, in which case it opens in a background tab.

That's still the behavior on Mint 22.2 and Firefox 146.0.1