I have observed an effect where new physical players and, to a lesser extent, software, sound better because I am somehow directed to "listen" to them without so much "knowing" how the player works and already "expecting" it to start outputting music, by which time listening seems sort of "redundant" already. It's difficult to put into words.
Opium's built-in player is poor. As has been reported earlier, Opium runs at lowered process priority and this will cause sound to stutter when the machine is under load (and the web browser itself causes considerable load).
Firefox's player is stable in the same conditions, and looks better. It's seek bar also reports on the playback position more precisely with an arrow rather than a circle which obstructs the bar.