You cannot legislate that someone must care about a societal/cultural/mental distinction which is sometimes binary and sometimes fluid and which has a non-circular definition linked to biology in the case of cisgendered people but a circular definition in the case of transgendered people and which doesn't exist for cisgendered people as anything other than societal expectations/stereotypes but isn't that for transgendered people, because then masculine ciswomen would be men, so it is something different that cannot be experienced by cisgendered people because they don't feel a disconnect between their mental and physical conception of themselves, but the disconnect isn't only physical because living as a woman is supposed to mean something more in the case of transgendered people even though this just brings us back to societal expectations and stereotypes that many women find insulting.
You cannot legislate that a business must not fire someone for a reason relevant to the job. Forstater was working for a think tank. Having publicly accessible discussions of a belief is within the job description of a think tank. If she did it contrary to her employer's guidelines, it's fair that she loses her job.