> An Emoji picker should be an OS-level feature.

This is an "OS"-feature, where OS means the GUI-Framework that Firefox is using to integrate with the DE.

> If the OS doesn’t provide the feature, then that is the OS’s decision to make, and the browser should respect it.

That's a very strange claim. Nearly everything in any app is something the OS is not providing; that's why apps exist in the first place, to enhance the environment.

> Why would I want text input in one app to have a feature that text input in other apps lacks?

Why should they cripple themselves willingly; just to align with others? Especially as this is a web browser, which has become the main way of interaction with a big part of the world.