If you want to play videos on a Linux system, you can always install Google Chrome, but if you want to watch videos in Firefox then, depending on your Distro, you have to add some plugins.
In Sabayon Linux, which is a Gentoo-Made-Easy distribution, you definitely can watch videos both in Google Chrome and Firefox. How, you ask?
How to Install Google Chrome in Sabayon
Simple as this: Open a terminal and copy and paste.
Firefox in Sabayon Linux
Open a terminal and here are the instructions:
Then exit and restart Firefox. Worked for me!