I think they'd spend more time interacting with web-based applications than traditional OS applications.
If that's the case, then Firefox is actually adapting to people's expectation of the UI.
For me, this still makes as much sense as Excel opening up its preferences as a new spreadsheet, or a 3D rendering program asking you to edit a 3D wireframe to change options. Content is content, application is application -- I don't understand why confusing the two could ever be a good thing.
When I use MS Windows I really notice this, installers, application control panels, window titles and controls - seem to be all over the place. It's a minefield. Linux desktop suffers this too with different GUI toolkits - QT, GTK etc. There's a part of you that just adapts and gets used to it. But ultimately it's a pain.
Compare the popular web browsers. Despite their convergence, the UI's are quite different.