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You can use flash block add-ons.
Referal Link : https://addons.mozilla.org/en-US/firefox/addon/flashblock/
Most of the videos that autoplay are going to be Flash, so if you disable Flash it will dramatically cut down on the number you experience. Open Firefox, the CTRL+SHIFT+A to get to your add-ons menu. Once you are there, look for Shockwave Flash and change the setting in the rectangular drop-down menu on the right to “”Ask to Activate.” This should stop the vids from playing unless you grant permission.