Security is always complex, and simple rules always have flaws. That doesn’t mean simple rules are always bad, it just means people who build systems around them do need to understand they need to do more than just blindly follow simple rules. I’m pretty sure you’ll find FF doesn’t alert on those sites, because they are being handled by a more complex rule than a vanilla low or medium traffic site.

Do you trust dropbox.com that they separate their servers, do you trust google that they do good job in terms of security and containing malware to single file? If yes then there is no need to include those.

On the other hand if you don't trust then those should be included.

Question is, does Mozilla trust google enough? Does Mozilla trust some random website where people host pirated content?

That is random article written by some random guy. Seems like he is more technical than average Joe, but he does not have any statistics to show why this behaviour was implemented. It just looks like a nag that he get his pirated downloads flagged by Firefox. It is backed up by bunch of people who also use it on /r/libgen. If they don't like it they can move to IE6.