If it makes your life easier and it doesn't cause worldwide chaos and madness, I guess it's all right then.
Firefox on Windows follows Windows GUI behaviour, since it's a Windows application. On RISC OS, an application rarely, if ever, uses more space than required to display a window's full content, i.e. it will not 'stretch' the content when toggling the size button to maximise a window. Anyway, I understand your point. It's always a matter of personal preference, but my previous comment mentions why I think it's "wrong", i.e. dated behaviour.