>Hint: The answer is in Marcuse. His theory of "repressive tolerance" holds that freedom of expression can only be afforded to the left. Right-wing (and perhaps insufficiently left-wing) voices and opinions must be quashed if we are to bring about a fair society
I cite Repressive Tolerance a lot, and it's best to read it as a directly political rather than theoretical text. I also find some of his reasoning quite convincing. Nevertheless, although Marcuse and Popper were academic enemies of sorts, the concept gets a look-in through Popper's paradox of tolerance. Marcuse goes into quite some detail surrounding the justifications and function (at least in his time) of the "free speech principle". Marcuse's argument for censorship is based entirely around his assumption that there is limited or little capacity for rational discourse in the public sphere. The title, as with the other two entries in the book, refers to the idea that tolerance itself contains an element of repression - not that we should be repressively tolerant.
Marcuse was also writing in the context of the state (and the establishment around it) having no hope of adopting truly emancipatory goals. This is somewhat under question today, at least more than it was in the 60s. Besides this, there are quite a few good philosophical arguments for the regulation of speech or even against a free speech principle as a whole. I'd be happy to share them (some are linked in some recent comments of mine here.) - I think Marcuse was right on the money in terms of his diagnosis. His prescription is debatable.
Finally, what is attributed to Marcuse and the Frankfurt School as a whole tends not to be based around the one essay which he wrote for a book on concepts of tolerance, but rather something much more abstract and dark: that the School existed to destroy Western civilization through progressive ideology. This is not only a shaky charge, but it has little to do with Marcuse. The right-wing attacks on the School today from the pundits mentioned earlier in this thread actually focus much more on Adorno, Horkheimer, and Benjamin, and even non-School thinkers like Gramsci.
And the claims that Marx was an anti-semite are almost comically overblown; there's plenty of scholarly work on this topic from both within and outside Marxism.