Totally a ubuntu beginner, for some site like https://learning.oreilly.com/ (it's an online book and video site for technical training and requires subscription) when I click to play a video, firefox always downloads the video instead of playing it. Youtube works fine though.
EDIT: Just installed the chrome and the site works fine on chrome i.e. video plays as expected.
EDIT2: The video format from the problematic site is mp4.