Many webmasters integrate videos on their websites, for example to finance their website. If you come across such a website with Firefox, these videos start automatically, which can be very annoying over time. In Firefox it is possible to disable the automatic playback of videos.
Firefox – Automatically prevent Autoplay videos from playing
Start Firefox and type “ about:config ” in the address bar.
Confirm the message " I'll be careful, I promise !".
Type " plugins.click_to_play " in the search box

Double click on the result and set the value to " true "

Then look for " media.autoplay.enabled " and change it to " false ".

Then restart Firefox.
Check if the videos start automatically now. If this is the case, please edit the "Shockware Flash" plug-in:
Start Firefox and type “ about:addons ” in the address bar.
Clicking on the Enter key opens a new window.
Click " Plugins " on the left.
Locate the Shockware Flash plug-in .
In the settings , select the “ Ask me to activate ” option .
The settings are saved by clicking on “OK”.
From now on, no videos will be played automatically.
UPDATE: to disable automatic launch of HTML5 videos do the following:
Start Firefox and type “ about:config ” in the address bar.
Type " media.autoplay " in the search box . double click on it so that the value changes from false to true

Restart Firefox