Should we go somewhere else to discuss, rather than heavily nested comments? Would a new discussion topic page be the right place?

I agree that the nested comment format is a little cumbersome (in fact, this is a bit of a complaint of mine about the LW format in general), but it's not clear that this discussion warrants an entirely new topic.

Terminology isn't terribly important . . . If you want to take the Popperian conception of a good theory and label it "justified" it doesn't matter so much.

Okay. So what is really at issue here is whether or not the Popperian conception of a good theory, whatever we call that, leads to regress problems similar to those experienced by "justificationist" systems.

It seems to me that it does! You claim that the particular feature of justificationist systems that leads to a regress is their reliance on positive arguments. Popper's system is said to avoid this issue because it denies positive arguments and instead only recognizes negative arguments, which circumvents the regress issue so long as we accept modus tollens. But I claim that Popper's system does in fact rely on positive arguments at least implicitly, and that this opens the system to regress problems. Let me illustrate.

According to Popper, we ought to act on whatever theory we have that has not been falsified. But that itself represents a positive argument in favor of any non-falsified theory! We might ask: okay, but why ought we to act only on theories which have not been falsified? We could probably come up with a pretty reasonable answer to this question--but as you can see, the regress has begun.