I have a discussion on this going on over at reddit. Their solutions did not help.
It isn’t just Youtube. It’s gifs on Reddit(and videos on Reddit). It’s facebook videos.
It’s not merely autoplay. I can’t even click the play button for them to play. Spinning circle, will not begin.
I want to completely down grade. I was on an ancient version of Firefox and finally caved.
I can not figure it out.
Windows 10, home edition.
Could this be related to The Great Firefox Fuckup of 2019? Did the problem start last night?
I upgraded this morning and yes, my addons did not work. They are back and up and running, though.
I have, however, done a refresh eliminating all my addons.
No change in the videos, though.
I found my adblockers and Amazon Assistant disabled yesterday (so opened Chromium instead)
Extensions restored a couple of hours later.
Everything worked but was littered with annoying adverts all over the place, something I normally never see.
Chromium unaffected but I see that Google have hinted that they will disable extensions, primarily adblockers.
Ubuntu 19.04 Disco Dingo and Linux Mint 19 Tessa.
All of my extensions are working. I can not play videos on any site. 
Not too sure what to suggest there except to try a different browser - Chrome or Opera perhaps.
Clear your cache, cookies, temporary files, internet files etc.
If you haven’t got it, try CCleaner but be careful where you get it from - I used to use filehippo.
I currently use Bleachbit (on Linux machines) but haven’t used Windows for years so don’t know how it would work on a Windows machine.
Either will need some configuration but aren’t difficult.
So you’ve already done a Refresh. That eliminates a whole host of problems. It’s not the fact that you upgraded from such an old version, or that your profile was broken.
You mention GIFs, but I note that many GIFs are not actually GIFs, especially the ones on sites like Reddit. They convert their gifs into much smaller video files. So the problem may just be videos.
Since you didn’t mention it, I’ll recommend the most basic: Restart your computer. Don’t choose Shutdown, but Restart. This will completely reload Windows. Then try again.
After that, first thing I’d suggest trying is disabling hardware decoding for videos. You can do this with a setting in about:config. Type that URL into the address and press Enter, and click through any “I’ll be careful” screen. Then type in media.hardware-video-decoding.enabled, and try setting that to false, and then restart Firefox. See if videos will play now.
If that doesn’t work, next would be to disable hardware acceleration altogether. Open your preferences, and type “Hardware” into the search. Then uncheck “Use recommended performance settings” and then uncheck “Use hardware acceleration when available” when it shows up. Restart Firefox to be sure, and try videos again.
If neither of those work, then I can only recommend an additional check: Open up Internet Explorer, and try to watch videos using that. If videos play there, then the problem almost certainly is with Firefox. The only thing I can recommend then is to try a different browser. Chrome basically has extension parity with Firefox, and it will import your bookmarks and stuff, so you may be able to use that to get back up and running.
Malware is not impossible, but this isn’t a typical presentation. But running a scan with your antivirus wouldn’t hurt, too.
It was only Firefox. Edge, Chrome, and Tor(I downloaded it just to check this) all worked fine. Zero issues.
I tried the about:config fix related to hardware acceleration. It…appears to have worked! It was set to true and I flipped it to false.
I will update you if it goes back to failing or come back to thank you even more if your fix becomes permanent!
I wonder…why does my laptop struggle with this? And why only on Firefox?
The GFF continued today.
There was another update today.
Yep, mine have all updated with no problems as yet, videos play normally.