A bugbear of mine is when switching flash videos such as Youtube or Vimeo to fullscreen in OS X on my retina MacBook Pro, it switches to low resolution none-retina video and controls.
I have a workaround, it may break some other plugins such as Silverlight, so be aware.
1. In Finder locate Firefox app, right click/double tap to open the context menu and select Show Package Contents.
2. Navigate to Contents/MacOS/ and find plugin-container and right click and Show Package Contents again.
3. Navigate to Contents folder and open Info.plist in TextEdit or your favourite text editor.
4. Add these lines:
5. Save the file, open Firefox.
Enjoy fullscreen retina looking Flash.