With reference to this post, kara pointed me towards: Jumpcut, which is
an application that provides “clipboard buffering” — that is, access to text that you’ve cut or copied, even if you’ve subsequently cut or copied something else. The goal of Jumpcut’s interface is to provide quick, natural, intuitive access to your clipboard’s history.
Seems to be working well. Not quite the one click FF plugin I wanted, but better than what I was doing previously, with the added advantage of working as a general clipboard enhancer, not just a FF bonus.