Many websites, especially social media sites, have taken to automatically playing videos that appear in your feed. The videos typically only start playing when they scroll into view, although this is not always the case. Autoplay is designed by site operators to increase user engagement. It’s hoped that at least some users will decide to continue watching a video they wouldn’t otherwise have played.
Autoplay can be an annoying feature for a number of reasons. Firstly, for users with limited data caps, automatically playing a video uses some of their data allowances, even if you never wanted to watch the video. In some cases, autoplay videos also automatically come with sound enabled. This can be particularly annoying if it suddenly blares loud sounds in an otherwise quiet environment or overlays other music that you’re listening to.
Firefox on Android offers the ability to override and disable autoplay in any website open in the browser. To be able to disable autoplay, you need to open the in-app settings.
To be able to access the in-app settings, you first need to tap the triple-dot icon in the top-right corner of the app.

Next, in the drop-down box that appeared, tap “Settings”, which will be the second option from the bottom.

In the settings, tap “Advanced”, the third option from the bottom, to open the advanced settings.

In the advanced settings, tap “Allow autoplay” under “Media” to configure if videos are allowed to automatically start playing or not.

There are two autoplay options, “Allow autoplay” and “Block autoplay”. If you select “Allow autoplay”, sites will be able to automatically start playing videos. Conversely, if you select “Block autoplay”, sites will be unable to automatically play videos.

Prime says
Please look for ‘site permissions’ in the settings. Be sure to remove or edit any site exceptions after you have blocked autoplay.
grumpyOtaku says
BZZZTTTT!!!!!!! No such setting in Firefox Android 102. Try again!