Interestingly, Firefox on Wayland renders the emoji correctly in the tab title, but the window title renders it as two rectangles and the male symbol. I assume this must be some difference between system fonts vs Firefox's fonts.

dang is definitely wrong about what “plain text” means in that thread.

Happy to wrong but when saying something like that, you should explain why and how, so we can all learn something!

Plain text is often contrasted with markup or rich text. UTF-8 text is neither markup nor rich text.