I don't think that's the case. I haven't heard PostgreSQL users trash Firebird or SQLite. The complaints about MySQL are rather specific and have historical roots regarding MySQL saying things like:
* you don't need transactions
* you don't need foreign key constraints
* you don't need ACID-compliance
At least the NoSQL folks tend to be honest about their products not replacing the traditional RDBMS approach....
Granted many of these don't apply anymore, but then legacies live on interaction-wise. This being said, I know very few real Pg users who harp on MySQL anymore. Most of us figure that Oracle does better advocacy on this area than we can.
lol @ "At least the NoSQL folks tend to be HONEST about their products not replacing the traditional RDBMS approach...."
More like, technically true, but contrary to the whole reason for doing what they're doing.
Nobody puts a lot of work into something "honestly" not intending to replace the thing it's replacing.
They say this to appease the people who point out that it doesn't do all the same things.
If I built a better alternative to a common item, you could bet your bottom dollar i would be very "honest" about it not replacing that common item. The last thing that statement would be is "honest." It's appeasement.
But in this case at least you are using PostgreSQL outside of anything even remotely purely relational. It is at least object-relational, and often key-value-id (like hstore) or document store (JSON or XML).
The power of PostgreSQL occurs from the power to ignore the relational model and take other approaches when it is helpful.