Update: In the latest version of Firefox 24, you can now fix this problem simply by making sure that you have selected “Enable hardware acceleration where possible” under Preferences, Advanced, General.
Some Retina Mac users will have noticed an annoying bug with Firefox 23 where Flash videos only play one quarter of the screen. This is a known bug in Firefox 23 on Retina Macs which Mozilla are fixing and will probably issue an update for in Firefox 24 due in mid-September. Until then however, you can follow these simple steps to fix it temporarily.To fix the Flash video bug in Firefox retina Mac that makes video look like this, follow the steps below:
1. Open a new tab and type about:config in the URL bar
2. Click on “I’ll be careful, I promise!”
3. In the filters box, type gfx
4. Double Click on gfx.hidpi.enabled and change the value to -1
5. Close and restart Firefox and the bug will be fixed and you should be able to watch Flash videos including on YouTube, normally again.
You will notice that text looks a bit blurry in your browser afterwards but this is unavoidable until a proper fix is issued by Mozilla. If you have any problems with this solution, let us know.