Though I am writing this in Firefox Developer Edition, I stuck it out with Firefox ESR while plugin developers caught up with the new extension model put in place for Firefox Quantum. Eventually, heavy memory consumption and sluggish rendering forced me to move along even if the addon bar continues to be missed.
During my time of sticking with Firefox ESR, I often turned to Opera when I needed something more speedy and it was then that I found videos playing automatically in loading web pages. Twitter was one offender as was YouTube though it is possible to turn off such things for the former by going to the Accessibility section of your user account settings.
In Opera, I wanted to have a more universal solution and found out where the Experiments section could be found. Though there is a need to realise that there may be changes in the future, entering the opera://flags in the address will get you there. Then, you need to look for Autoplay policy unless you have used the more direct address of opera://flags/#autoplay-policy. After that, you need to change the setting to Document user activation is required and that should sort things. If not, a browser restart will complete the task.