Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

Separate Issue; Your System Details shows;

Extensions

YouTube Flash Video Player 64.0.0

YouTube™ Flash® Player 2.0.2

YouTube no longer uses flash. Also, Having more than one version of a program may cause issues.

These should be removed.

Firefox will not allow the game to run; I'm not on YouTube

Please explain the problem in detail. What happens? Error messages?

Please provide public link(s) (no password) that we can check out. No Personal Information Please !