Night mode turns the background into black and the text to white (or whatever color you want, you can set it in Options).
Resource sniffer: similar to "network sniffer: – you can download and save almost everything that is being displayed or played by the browser, including .swf files, youtube videos (and videos of other sites), audio files etc. As a result, you do not need programs such as youtube downloader etc.
Browser id is the how a website sees what OS and Browser you are using. For example, if you would like to see the mobile-phone-version of a webpage, you can just change the browser id to Android 2.5 or iPhone, etc.