I think there are problems in the Mozilla OS/2 widget code, namely that the apps don't have a proper parent window, or is that a root window.
I see it mostly in SM, where the program object often loses track of SM and sometimes will open another app. Had it stuck on bringing TB to the foreground the other day, finally had to launch SM from the command line.
Mozilla is not a native application on any platform, just pretends and while the window frame and menu bar are native, most of the rest is a mixture of DOM, XPI, XPCOM and JavaScript.
I'm no expert and may be getting things wrong.