OS X 10.9 Fullscreen on second display doesn't stretch to full screen, but only part of it
Here's my workaround: right click on the 1/4-screen video and, if you're using Flash 15.0.0.xxx, you should see a Stretching is &lt;type&gt; item in the options list. Click on that and the video will fill the screen. Keep repeating until you get back to the 'Stretching is none' setting, unless you prefer something else.

I have the same issue when using firstrowsports.eu on my 2014 MBP, multiple different browsers, no plugins, no additional screen, hardware acceleration on and off, Mavericks and now Yosemite.