White: a layoff for a legitimate business reason (termination for convenience). Firing people who break the law or do things that are unethical (termination for cause). Firing people who fall short of a published and objective performance standard, if the policy is enforced uniformly. These are all considered to be unambiguously legal and ethical cases of termination.

Black: firing those who intend to unionize, firing with discriminatory intent, retaliation (including against protected insubordination such as whistleblowing), firing people because of disabilities that can be reasonably accommodated, firing people because they disclose compensation (anti-unionbusting provision), firing people for making legitimate use of HR or internal-mobility processes (a subcase of retaliation), firing people in retaliation for opposing harassment.

Gray: firing people who fail socially (when there's a record, in the company, of allowing internal transfer), firing people for ethically justifiable (but not explicitly protected) insubordination, firing people whose health issues interfere with the job, firing people you don't like.

In the US, most Gray cases are not considered tortious. You're correct. The burden of proof is technically on the employee to prove that it's a Black case.

If he knows what he's doing, however, the employee can establish enough Gray-Blackness that the company will desire to settle.

Typical firing example: Bob doesn't get along with his boss, Mark, who suspects Bob would rather be on another team. Mark gives him shitty performance reviews and he's flushed out. Looks like a non-tortious Gray case; the performance assessments are inherently subjective, but there's nothing about this that entitles Bob to legal protection. Even an inaccurate performance review is non-tortious managerial incompetence, right? Well, it's not that clear. For one thing, most companies make performance reviews part of the transfer packet. Then, a negative review can be construed as harassment: interference with work performance. Part of the employee's job is to cultivate internal relationships and that makes it harder.

Let's say that the performance review happened on February 1, with Bob being fired on March 15. Bob establishes that, in fact, he met with Alice (manager of another team) on January 24. Bob was, in fact, discussing internal mobility. He has a chain of emails to prove what was discussed.

That's not an uncommon thing for people to do when they don't get along with a boss, nor is it uncommon for bosses to pre-emptively fire flight risks. Now, Bob has a legitimate case for retaliation. He was making legitimate use of internal mobility processes and trying to build relationships within the company, which can reasonably be considered part of his job description, and he was punished for it with a negative review.

If, as is typical, his performance reviews are part of the transfer packet, his negative performance review constitutes harassment insofar as it interferes (tortiously) with his relationship with Alice.

Bob's claim is much stronger, if he has the paper. If he emailed himself (because, after being fired, he's lost access to his work account) copies of the relevant documentation, he has enough paper to initiate a discovery process. Even that, most companies would prefer to avoid.