Did you check the volume level of the Flash player on the YouTube site that you haven't muted the player? See: * https://support.mozilla.com/kb/No+sound+in+Firefox * https://support.mozilla.com/kb/Video+or+audio+does+not+play You can click the Volume icon on the Windows Taskbar near the clock and click the Mixer link at the bottom to open the mixer.<br /> You can adjust the volume level for each application in that window, so make sure that Firefox is not muted or set at a low level.<br /> Current Firefox versions use the plugin-container process to run plugins, so you need to check the volume setting for the plugin-container process.<br /> You will only see the Firefox program or the Plugin Container in the Mixer if you are on a website that plays sound.<br />