I watched the presentation in Pycon 2015[1], and I really liked how honest/aware they are about the challenges and tradeoffs (so far, the performance it's pretty bad compared to javascript, and the download size is still an issue). At the end is answered why python can't be directly included in Firefox or other browsers(the presenter/main developer works for Mozilla)
[1]https://www.youtube.com/watch?v=PiBfOFqDIAI