@ThomH

Firefox uses native theme engines wherever possible so what you see *is* native OS rendered. On Windows for example it uses the uxtheme.dll to render the scrollbars & buttons through the same code as any other app. Similar happens with GTK & OS X. It also uses things called overlays to ensure that layout of menus and dialogs conform to system norms. For example GTK & Windows put their dialog buttons in a different order. Peripheral dialogs such as file, print selectors are also native.

So the look is native but the "feel" may not be. A Firefox button might look like a regular button, but the actual click behaviour is handled by Firefox code, not the OS. Firefox has a cross-platform widget/ subsystem that sits over the native event model. Operating systems / Widget layers may have very subtle behaviours implemented into their standard controls which Firefox doesn't emulate perfectly. For example, scroll bar tracking might differ, or accessibility, or support for tablets etc. On the whole though I consider Firefox to be one of the native feeling browsers of all of them, including IE.