Instead of going through a refresh and setting everything up again : Please give this a try : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 58.0.2 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

Could you also Please check this url and see what it tells you : https://www.youtube.com/html5/

Please let us know if this solved your issue or if need further assistance.

So I did everything you said, and at first it fixed the issue, but when I restored my Firefox profile, it went back to doing the same thing.

So I figure it probably something to do with my profile, but is there an easy way of figuring out what in my profile is doing this?

Ok, never mind my previous post.

I figured out whats going on. I freshly installed Firefox and the second I sign in to Youtube, even on a brand new profile, the issue starts. Not signed in I can use Youtube fine. Being logged in seems to be the main problem.

And this is not exclusive to Firefox, I just did some checking and this happens on all my browsers.

lordgaspode said

And this is not exclusive to Firefox, I just did some checking and this happens on all my browsers.

That makes no sense I have FF58.0.2x64 and all other Browsers load and play youtube just fine. What it sounds like is you got a over active A/V or Firewall that is actively blocking sites is what is going on.