Install Flash Player for Watching Video in Ubuntu 16.04 Xenial Xerus (LTS), Ubuntu 15.10 wily werewolf, Ubuntu 15.04 vivid Vervet, ubuntu 14.10 Utopic Unicorn, Ubuntu 14.04 Trusty Tahr (LTS) and Other Ubuntu Derivative System
You can go to Software Centre > search for Flash > select the second entry on the page which is 64 bit Flash version.
Go to Youtube in Firefox and run any video. It will prompt you to Install missing plug-in since you do not yet have Flash plugin installed. Click the prompt and follow its process to enable Flash in Firefox.
Go to terminal and run this command. It will fetch Flash and other miscellaneous plug-ins for Ubuntu.