does anybody know what needs to be installed to play videos from www.primevideo.com on chromium or firefox in Ubuntu ?
Applies to firefox: type "about:preferences" (without quotes) in your address bar. Scroll down until you reach "Digital Rights Management (DRM) Content" and check the "Play DRM-controlled content" box. This should enable you to reproduce videos on primevideo, netflix and music from spotify among others.
I also encountered a similar issue with Firefox 61 on Ubuntu 18.04. It appears that you require mp4a codec to be installed in your system before you can play videos on primevideo. You can install these by:
$ sudo apt install ubuntu-restricted-extras
You might need to restart your browser after the installation.