Justificationism doesn't even have its own Wikipedia page, what more do I need to say :) I know, inductive reasoning does. My point is just that regardless of what other challenges to Popper are out there, Kuhns' is possibly the most well known.
As far as details on how Kuhn challenges Popper, I'd recommend you read The Structure of Scientific Revolutions to find out--specifically chapter 8 where he addresses falsification and chapter 12 where he mentions Popper by name. Popper is also mentioned in the postscript.
In chapter 12 Kuhn mistakenly says that Popper is seeking falsifications that necessitate the rejection of a theory. He then points out that falsifications are imperfect, so that is problematic. That criticism would be correct, but Popper is not seeking to necessitate the rejection of any theory. Popper is a fallibilist who does not seek perfect or final falsifications, nor does he seek to justify or prove falsifications. So in short, Kuhn says there are certain difficulties, which he is correct about, and he says Popper's theory faces them, which he has wrong on account of misunderstanding Popper's theory. BTW Popper was aware of the same difficulties Kuhn brings up, and sometimes uses them in his arguments (they are not problematic for Popper's conjectural knowledge approach, but they are problematic for many other approaches).
In the postscript section 5, Kuhn says communication is always partial, so in a debate we can never prove the other guy is wrong in a way he must accept. This is the same sort of thing as before, and it is basically correct, but Popper is aware of it and his theories are compatible with it. One way Popper and Kuhn do contradict in this arena is about how valuable partial communication is. Kuhn is pessimistic about partial communication across paradigms, but Popper is more optimistic and says by an effort we can learn from each other even across paradigms/cultures/frameworks (see Popper's _The Myth of the Framework_, title essay).
I looked through ch8 also, but didn't spot any criticism of Popper's falsificationism. If you are still convinced it is there, please explain it in your own words or site specific paragraphs.
PS regarding justificationism, the prevailing idea of knowledge is that it is justified true belief. This can be found in the dictionary or any intro philosophy textbook.
PPS It is unfortunate that PG's nested thread rate limiting is hiding the reply link on this comment. I think this is a perfectly good, respectful thread. So it's collateral damage? :(
As far as details on how Kuhn challenges Popper, I'd recommend you read The Structure of Scientific Revolutions to find out--specifically chapter 8 where he addresses falsification and chapter 12 where he mentions Popper by name. Popper is also mentioned in the postscript.