If memory serves, the system style stuff was a bolt on that Dave Hyatt did a bit later than the core work, and mostly for Windows.
The one theme for all platforms that was initially envisioned by Netscape kinda sucked ("Blue" for anyone that might remember.) And when Ben Goodger began his native-looking theme for Windows (pretty sure he started with preferences, maybe even before he got hired,) Dave got to work on Windows OS style queries and hooking OS style into XUL so the Mozilla suite's emerging new default theme could reflect those in its widgets. That work was ongoing even after most of the rest of XUL had been settled and I think Dave was still working on that even at Apple, right up until Mozilla decided Firefox would also ship on Mac and Linux and not just Windows* (as the several of us working on it has envisioned.) Once Firefox was running on Mac, Dave was precluded from continuing because of his Safari work. (He'd gotten permission to keep at when it was not directly competing with his Apple work.)
* Firefox was, for about its first year, meant to be Windows only. Ben Goodger had recently built a .Net wrapper for Gecko called Manticore, and there was Hyatt's Chimera for Mac, and Marco's Galeon for Linux, so the thinking was the major platforms would be covered with great standalone Gecko based browsers (no mail, HTML authoring, chat, etc.) But then Ben abandoned Manticore. Dave was convinced that XUL could actually kick ass on Windows (though also certain it could not on Mac) and so Ben started mozilla/browser to be the standalone Gecko browser for Windows. Ben also quickly abandoned that project and Blake Ross, an intern I'd recruited to Mozilla the year before, picked it up. He and I conspired on it with several others joining the effort during the end times at Netscape. Bryan Ryner stepped up to make it work on Linux, for example. staff@mozilla.org, well 5 of us I think, were sitting at a picnic table outside of Netscape's Bldg 21 when we decided that after Netscape wound down, we'd transition from the Suite to standalone browser and email (then called Phoenix and Minotaur). Cross platform would thus become a requirement. We weren't going to drop Seamonkey without replacements for it on Mac, Windows, and Linux. It took a while to get the Mac version up and running and as soon as that happened, Hyatt had to bail. About a year later, we released Firefox 1.0.