So there's two sets of things you'll need if you want to get your Firefox-derivative looking like a proper windows app. First, you'll need this list of extensions. Second, you'll need to write a user-style that looks something like this:
#navigator-toolbox {
background-color: transparent;
margin: 0px 0px 0px 0px;
}
#browser-panel {
margin: 0px -1px -1px -1px;
}
#titlebar-content {
background-color: white;
margin-left: 0px;
margin-top: 1px;
margin-right: 1px;
}
#TabsToolbar {
background-color: white;
margin-top: 0px !important;
}
#toolbar-menubar {
background-color: white;
}
After that, you should be good, just might need some tweaking. But this is just me, you may like it looking another way. So go ahead, explore, and try new things!