Firefox doesn't have this feature at all.
I was curious as it seems like a nice thing to have and I use Firefox, so I looked it up. It's been suggested and you can upvote it here: https://connect.mozilla.org/t5/ideas/text-wrap-pretty-for-de.... Per the last post there (Aug 24).
> Mozilla standard position on this feature is positive > https://github.com/mozilla/standards-positions/issues/993