Popper's point is that if someone wants to curtail your right to free speech, you have the right to curtail theirs. On the other hand, if a political opponent respects your right to speak freely, you should do the same for them. It's not really a paradox; it's about symmetry.
By analogy, imagine if someone commits murder. Would putting them to death as punishment also be murder? No. You have the right to life as long as you respect other people's right to the same.
The problems always begins with grey and ends in black. Who defines when, where and how someone is curtailing their right to free speech? There is always asymmetry in power.
By analogy, imagine if someone commits murder. Would putting them to death as punishment also be murder? No. You have the right to life as long as you respect other people's right to the same.