To expand on this, there are two settings you can put in user.js
/ prefs.js
(desktop) or via about:config (mobile), documented on the Mozilla Wiki:
user_pref("media.autoplay.default", 5);
user_pref("media.autoplay.blocking_policy", 2);
Two bonus settings if you want to get rid of the "do you want to enable DRM?" pop-in bar when hitting one of those sites:
user_pref("media.gmp-widevinecdm.enabled", false);
user_pref("media.gmp-widevinecdm.visible", false);
hth
A case in point - I enjoy playing a specific game (Destiny), the subreddit has 3.25 million subscribers and at any given time 500-1000 active users. There is just nowhere else to get the same amount of information (bugs, tips, latest news, etc.) than this community. If the UKJobs subreddit is like this, OP just needs to use it for what it is in their time of need.