In the abstract, Popper’s claim is arguably valid. However, it’s routinely used as an excuse to censor disagreeable opinions that have first been demonised. When I was at university, and I argued for the toleration of racial prejudice, the quote was rolled out, and I was asked if I’d have argued for the toleration of the NSDAP in 1932. The answer, of course, was that a few dozen NF intellectuals and their skinhead followers were not in the slightest position to take over in England: the main threat to liberty was proto-PC. The same argument is now used to shut up anti-gay evangelists and whatever has become of the BNP.
The argument should always be turned into who is a clear and present danger to a liberal constitutional order, not who may or may not be “intolerant.”