Regarding comments made by John here:

What's the explanation for why Firefox is such a user-experience success given that it's open source (OS), and OS typically is not a conducive model for high quality, polished (complex-) software?

It's the only complex OS software that rivals high commercial quality (that I know of anyway). Open Office and Inkscape, while functional and capable, are relatively ugly messes and don't rival commercial equivalents in terms of user-friendliness and attention to design.

I'm guessing the way Mozilla handles things has something to do with it, and I'm guessing XaraLX would have been handled in a similar professional, controlled, structured, intelligent way by Xara to avoid the standard programmer soup. But are there other factors?