I made a kickstarter video and uploaded it to their site. When I view it on Firefox it looks very pixelated. When I view it on Safari, crystal clear. It's a quicktime video, 1920X1080, H.624 and 5000kpbs. I'm on a Mac OSX and using FCP6 to export.
I did clear my cookies and cache on Firefox and made sure all my plug-ins were up to date. And I asked friends on other computers to check both browsers and they had the same results.
Which leads me to believe it's something about the way I encoded the material and the way Firefox is reading it.