Go ahead, blame dodgy government censorship on FF
Quote from that very bug post ... "The operation of the WhatsApp Web application was just tested on another phone number, the same operator, in both browsers of the latest versions Mozilla and Google Chrome and it was established that this is an artificial barrier for this phone number, either from the mobile operator or from the special services of the country that can persecute me as a citizen fighting alone against corruption, repression of the unwanted, lawlessness, violation of human rights and freedoms and violations of democracy in the country."
Nothing to do with FF at all.
The guy in the post had the decency to explain the real problem, despite which, SJVN still quotes it as example of an FF problem.
But yeah, go ahead and blame Firefox because that suits your preconceived idea.