I use Firefox's own password manager (https://www.mozilla.org/en-US/firefox/lockwise/) inbuilt into the browser. I think it's a better compromise than having to expose sensitive credentials into my clipboard. IT works across devices too, so there's no copy pasting on my Android device which god forbid is probably reading my clipboard too (I know Facebook used to do that).

I do not see a reason to copy paste in any use case at all.