Maybe Opera is using the TrueType instruction engine in freetype2 and Mozillish browsers aren’t? Or maybe it's the other way around.
BTW I need to figure out how to get Flock (and other Mozillish browsers) to let me type Esperanto letters in a composing box. In Opera it’s no problem. Flock et al. let me type a circumflex "a" (â) but what about a circumflex "c"? No go.