If you want to prevent videos from autoplaying in your Firefox , you can disable HTML5 Autoplay and Adobe Flash Player ​. 

1. Open Firefox .
    
2. Type about:config in the Address Bar and the hidden configuration settings will load.
3. Now search for the media.autoplay.enabled setting. Change that setting to False.
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Secondly you should block Adobe Flash Player ​ to stop Flash  videos from autoplaying.
 

1. Open Firefox .
    
2. Click on the Menu and click Add-ons.
   
    
3. Click on Plugins and then click on Ask to Activate.