It's worth copying and pasting the quote used on Wikipedia, because it actually espouses a very strong view that I think many Americans, even moderate ones, might find surprising:
Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.—In this formulation, I do not imply, for instance, that we should always suppress the utterance of intolerant philosophies; as long as we can counter them by rational argument and keep them in check by public opinion, suppression would certainly be most unwise. But we should claim the right to suppress them if necessary even by force; for it may easily turn out that they are not prepared to meet us on the level of rational argument, but begin by denouncing all argument; they may forbid their followers to listen to rational argument, because it is deceptive, and teach them to answer arguments by the use of their fists or pistols. We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant. We should claim that any movement preaching intolerance places itself outside the law and we should consider incitement to intolerance and persecution as criminal ...
Unfortunately, this argument and its result is not a good guide, because it is vague of what is "intolerant" and easily gets interpreted as fighting fascism with fascism. In order to win, we must become the enemy, because we are better. No thanks.
That is very kantian. If someone came to your door and asked you to point to where your children were so he could murder them, would you tell the truth just because lying is wrong?
Similarly, would you tolerate hate speech that could put your life in jeopardy just because you believe in free speech?
The point I believe is not taking the ideal to its maximum blindly, but clearly defining boundaries you can't cross, even if it meant behaving intolerantly.
That part is clear. What is not clear is how does he determine who is intolerant enough to fall in that category. And also, what that proposal of righteous intolerance means. Does he stop listening to them? Does he de-platform them? Does he incite the mob against them?
No, you’re just being facetious. The problem is that for yall, everything needs to be sacrificed at the altar of free speech, even reading comprehension. This is exactly what Popper is saying: we can’t tolerate the intolerant because they don’t play by the same rules.
Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.—In this formulation, I do not imply, for instance, that we should always suppress the utterance of intolerant philosophies; as long as we can counter them by rational argument and keep them in check by public opinion, suppression would certainly be most unwise. But we should claim the right to suppress them if necessary even by force; for it may easily turn out that they are not prepared to meet us on the level of rational argument, but begin by denouncing all argument; they may forbid their followers to listen to rational argument, because it is deceptive, and teach them to answer arguments by the use of their fists or pistols. We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant. We should claim that any movement preaching intolerance places itself outside the law and we should consider incitement to intolerance and persecution as criminal ...