In order to change your Firefox Configuration please do the following steps :

1. In the location bar ], type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise! to continue to the about:config page.
3. Search in the top line by tapping next to the microscope "Autoplay" and tap on "media.autoplay.enabled" to change it to false.

However this is also in the Settings Menu of your Firefox for Android Settings.

1. Tap on the three dot menu
2. Tap on "Settings"
3. Tap on "Display" and then tap on the check box labeled "Allow autoplay Control if websites can autoplay videos and other media content"

All done