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

For Firefox
 

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.
   
   ​

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.
   

For Google Chrome 

For Google Chrome, please follow the instructions on this FAQ. 
​

For Safari 

If you want to Prevent Videos from Autoplaying in Safari , you can disable HTML5 Autoplay and Adobe Flash Player ​. 

1. Open Safari  and navigate to the Preferences pane.
    
2. Select the Websites tab and then Autoplay.
   
    
3. At the bottom of this window you can choose to Disable Autoplay for Content with Sound, or to Disable Autoplay Completely.