How do I override that blacklist and forcefully whitelist that certificate?
My go-to would be something locked down in a VM if at all possible, given the nature of the threat.
Also I really dislike that Mozilla wants to decide who should I trust. If I added that certificate, it's my business, not theirs. They should encrypt bytes and display HTML, not engage in foreign politics.
Presumptions of user autonomy, consent, or informedness are invalid.
Which means that the bypass process should be highly inconvenient.
I've addressed that separately in a direct response above -- might not be a solution for you, but it's the direction I'd look to. Alternatively, you could look for what Firefox's behaviour in the presence of locally installed certs would be, though as noted above, given the threat model, it largely shouldn't do that.
Keep in mind that a large chunk of Mozilla, Google, and Apple's stance here (and I suspect Microsoft will join them) is that this is a very bad practice of CAs or governments, as not only will the browsers flag this practice, but those certs and a lot of collateral damage will result. This is by all appearances deliberate and a strong message to not do that then, to any governments which are considering similary asshattery.
And failing to respond forcefully to such actions and threats risks compromising all trust whatsover in the browser and CA models. Which are rickety enough as it is. So Mozilla, Google, and Apple most definitely have dogs in this fight as well.