I am a big fan of Chrome’s webrtc-internals tool. It is one of the most useful debugging tools for WebRTC and when it was added to Chrome back in 2012 it made my life a lot easier. I even wrote a lengthy series of blog post together with Tsahi Levent-Levi describing how to use it to debug issues recently.
Firefox has a similar about:webrtc page which shows the local and remote SDP for each page as well as a very useful grid of ICE candidates. But unlike Chrome it does not show the exact order of API calls or nice graphs obtained from the getStats API. I miss both features dearly. Edge and Safari don’t support similar debugging helpers currently either.