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.
After trying a number of things, finally installing Ubuntu Restricted Extras package and restarted Firefox fixed the issue.
See here about how to install the package.
Hope this helps others who are like me just starting to use Firefox on Ubuntu.
If you have installed Chrome and the video is playing as expected it could be that the web browser that were using before is set to save video files instead of playing them on the web browser.
For instance, in Firefox, there is a setting (under Preferences) General > Applications to specify default action for certain content types.
A similar question has been asked in Mozilla support, although the question pertains to Windows the solution is applicable to Ubuntu as well. You can reset Firefox Profile (if you are using Firefox) by uninstalling and deleting the profile directory (/home/USERNAME/.mozilla/ or ~/.mozilla) and then installing Firefox again. Note: You should make a backup of your Firefox Profile. You will also need to reinstall all the add-ons that you have installed before.
