I'm on the side of this being an unintentional effect.
I'm too under the weather to dig into this, but this might be a mismatch between Firefox and the spec. I don't see in the spec [1][2] where this character could be removed since it shouldn't count as whitespace for whitespace processing.
It looks like in Chrome `innerText` contains the override. And the innerText spec is only 6 or so years old (!) so it wouldn't be too surprising if there were was a lingering incompatibility.
[1] https://html.spec.whatwg.org/multipage/dom.html#the-innertex... [2] https://drafts.csswg.org/css-text/#white-space-processing