I read recently a comment by a philosopher that Karl Popper's "falsifiability" theory is considered obsolete. Is this so? I always found it to be quite useful. If it's obsolete, what rendered it so, and by what was it replaced?

I'll add a third problem to Popper's views... it classifies obvious psuedo-sciences as sciences such as astrology, so long as they make potentially falsifiable predictions. Furthermore, it does nothing to distinguish something radically implausible like astrology from something more plausible, but not falsifiable such as ad-hoc psychological analysis. Popper's views and others similar to it (verificationism and logical positivism) belong to an era of philosophy when it was believed philosophy could be made 'scientific'. It has not really been replaced because few philosophers still hold to that belief.

I've seen some people romanticize about philosophy in melancholic terms, as if it's a "symptom" of the depressed and sensitive minds to do philosophy. Is this generally true? Does the intricacy of philosophy require to some level quiet reserve and conscientiousness rather than an outgoing personality?

I don’t think there’s any necessary connection between doing philosophy and being ‘depressed and sensitive’. However, philosophy constantly puts one in a position where our society’s most cherished beliefs and assumptions are questioned. This can be quite depressing if you let this lead you to the edge of nihilism. I don’t think philosophers are generally dark and depressed, but I do think there are enough instances that confirm this stereotype of the philosopher to many people. Of course, there are also ‘happy philosophers’ who think they’ve figured out something very encouraging about the nature of the world. Similary, I don’t know if the intricacy of philosophy requires a level of ‘quiet reserve and conscientiousness’ that is greater than other academic disciplines in the humanities. One of the hard parts of the job for many philosophers is the enormous amount of alone time needed for writing and research. But, I think that would be true of any research oriented job that required a lot of reading...