My first instinct is to not do this. It seems light/dark mode is something the user should set for the OS, knowing that setting will affect interfaces and content throughout their experience. Setting the Firefox theme doesn't seem like something that would change the experience of website content. We'd have to change user expectations.
(We want to debate this from the point of view of the users, not the technical perspective of the browser engine.)
But actually, I don't know what web designers would think of this, so I asked the question in a Tweet (super-unscientific-research). https://twitter.com/jensimmons/status/1103007418588454914
Another thought — currently Firefox defaults to the dark toolbar, the dark theme. So would we ship Firefox defaulting all websites to dark mode? Or would we change Firefox's default to the light theme? It seems like a severe choice to default (any website that has a dark mode) to dark mode, even for users who have not set dark mode in their OS, and likely haven't even heard of dark mode. It could create a lot of confusion to update Firefox, and suddenly some sites have dark background and light text.