The solution for Firefox (Firefox) cannot use HTML5 player under Ubuntu:
The Firefox that comes with ubuntu does not have a video player, so you don’t need to download flash (will not support it soon). Now many domestic video websites (B station, etc.) support HTML5, enter in the terminal:
sudo apt-get install ubuntu-restricted-extras
After executing the statement, you will enter the dialog box and click Next (not difficult, it was solved a long time ago, there is no screenshot, you can ask me if you have any questions)
Of course, you can download Google Chrome, I hope it can help everyone.