I am not sure this is a widely reported fault (I will look later). Probably coincidence. I would suggest as a first attempt to cure this you clear cookies for the YouTube site.

• How to clear Clear cookies and site data in Firefox_delete-cookies-for-a-single-site
• general info Cookies - Information that websites store on your computer

Presumably you are able to manually set the resolution where High Definition is available on YouTube video. As I understand it you are saying the problem is it defaults to Lo Resolution,not that HD will not play,

Also I wonder if you are using and seeing HTML5 or FlashPlayer videos. (LastI checked HTML5 was a bet option on YouTube site))

Notes to self

• HTML5 changes in Fx22 (& Fx21)http://www.mozilla.org/en-US/firefox/22.0/releasenotes/
• IIRC also H.264 available natively, (but prob not affectingYou Tube)

Yes, you are correct: Videos are defaulting to low res, but if manually set HD will play (but in the beginning, my videos used to default to HD). HOWEVER, HD takes FOREVER to load and will often freeze when playing. This never happened when I first installed Firefox either. HD played easily and smoothly.

I use Adobe Flash player to view YouTube videos.

I cleared all the cookies like you suggested (thanks for posting links to that too :) and it seems to have improved things a little, but the videos still don't look or default to HD. They are still noticeably fuzzy :/

Thanks for your response!

Could you try Firefox's Safe Mode? That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

This is some standard guidance that addresses the most common issues with Flash -- perhaps it will help here?

(1) Disable hardware graphics acceleration in Firefox and in Flash

(A) In Firefox, un-check the box here and restart:

orange Firefox button (or Tools menu) > Options > Advanced > General > "Use hardware acceleration when available"

(B) In Flash, see this support article from Adobe: http://helpx.adobe.com/flash-player/kb/video-playback-issues.html#main_Solve_video_playback_issues

(2) Disable protected mode (Win Vista & Win 7 & Win 8)

See this support article from Adobe under the heading "Last Resort": Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?

Thanks Jeff, I neglected to follow up on this one.

Try to clear the Flash local storage and settings.

• Windows Control Panel > Flash Player > Storage: Local Storage Settings
  • Remove a specific domain: Local Storage Settings by site
  • Remove all domains: Delete All and Delete Data

Flash Website Storage Settings panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

Global Storage Settings Panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager03.html