The error I meant is the message displayed at the top of the firefox, said libavcodec is vulnerable and expired (for some reason). After googling and find the article in http://askubuntu.com/questions/851190/how-to-update-libavcodec-on-ubuntu-14-04, the problem is libavcodec is disabled as default.

Libavcodec is needed to play the streaming in the website. Except for youtube. I still can play youtube with libavcodec disabled. I wonder why.

Anyway the solution is make the libavcodec enable.

Here is how

type about:config in the address bar, enter
click “I accept the risk” search libavcodec
change media.libavcodec.allow-obsolete;false to true