Firefox's engineers have done a remarkable job at trying to mimic the native context menu as much as possible, but it's not a 100% match (as evidenced by your discovery that it doesn't support dark mode). There have also been times over the past two decades where macOS updated something about the context menu that then make the Firefox one not match. The main point is that it really should just be a native menu; they shouldn't be spending time trying to make it match 100%.