Hi shyamalc, Try turning off Hardware Acceleration

Go to: Menu -> Preferences In the search box type hardware de-select Use recommended performance settings de-select Use hardware acceleration when available

I already tried that. The flashing still happens.

Does this happen with only Firefox in general, without even a page loaded? (E.g., the blank new tab page, about:blank.) Does it happen on certain pages, or only video?

One cause of various issues for Mac is FF not installing properly. Can you download a fresh copy of Firefox, then delete the current Firefox app. Then open the firefox.dmg and manually drag the firefox.app out of it, and to your applications?