This irks me - it seems that for no reason, all of my browsers suddenly auto-play any youtube videos embedded on a blog or a website.
Firefox ::: type about:config into the firefox location (address; URL) bar (confirm the info message in case it shows up) & search for the preference named plugins.click_to_play. double-click it and change its value to true.
Chrome ::: type chrome://chrome/settings/content in the address bar and enter. After the settings page appear, scroll down and look for the Plug-ins section, and change the option to Click to Play.
Opera ::: go to Preferences, select Advanced tab, look for the Content section at the left bar, check option for Enable plug-ins only on demand then click OK.
Other browsers; more tips ::: please leave them in comments!