I thought Firefox had the shockwave-flash plugin installed by default? I don't remember having to install it and I do have 11.2 on Firefox 20-- add-ons manager. I have no problems at all with flash, and I have flashblock installed. I watched my first ever Facebook video yesterday, it was flash. It worked fine. Maybe it isn't the browser. Maybe it's the desktop with all its specific packages or files. I have only tried Cinnamon, KDE and Unity with Compiz(I think-- whatever goes with Compiz). KDE has been the best of all, for me, on Ivy Bridge. Desktop does matter.
i5 3570k, ASRock z77 Extreme 4, 8 Gb Ripjaws 1600, Antec 430w psu, HVR 1600 tv tuner, custom case- marble top, oak face. Carver & DCM Time Window sound system. Mint 14 KDE.