Youtube's 1080p Option Is Unavailable To Me In Firefox

I have spent some time trying to track down a reason for this and, failing to succeed, I am posting the question here.

YouTube is a source of near-constant problems for people because cell companies and internet service providers hate it. They hate it because it's so popular and sucks up a lot of bandwidth. Because, remember, telecom companies are generally evil, and their ideal scenario is where you give them $100 every month and then never use their product.

But I digress. I have three computers in my house, two on WiFi and one CAT-5'd into the wireless router provided by Verizon. The two computers on WiFi always provide the option of 1080p in videos that have it, but the computer that is hardwired never does when using Firefox. Chrome sometimes won't show it, but Internet Explorer will always give me the option.

I thought it was my cookie blocker, my ad blocker, my script blocker, or my security suite, but the issue persists in Firefox with all of those programs turned off. I am at a loss.

UPDATE: SOLUTION FOUND

I discovered the problem. I have a few accounts connected to Google, and a few computers, and this led to some confusion.

On one account, I had opted into the HTML5 video beta that is available at the bottom of the YouTube page. On others, I was either not signed in, or had not opted into the HTML5 video beta.

YouTube's HTML5 video option does not currently support 1080p videos. They are only available through the Flash video player.

If 1080p videos are mysteriously not available to you, click "try something new" at the bottom of the page and opt out of HTML5.