I've looked through related posts here and have been unable to solve my problem. I'm running Yosemite on a new Mac Air. I have FlashBlock, Ad Block, and NoScript installed. I had all of those on Firefox on my previous Mac running Mavericks, with no problem. When I'm on youtube, I can't play videos. I get the black screen with the cursive "f" in a square in the middle of the screen. Normally, if I click that square, the video should load and play - but no luck. YouTube content embedded in other sites - for example, on a media link posted on Twitter - plays with no problem. If within that player I click on the YouTube icon - which should open YouTube in a separate tab to play the rest of the video - I do get the new tab, and it opens the right page on YouTube, but again, it won't play. I've attempted to fix this by: - reinstalling of Firefox; - updating Firefox; - disabling all add-ons - restarting Firefox in safe mode. Nothing gets YouTube to work. I can watch it in Safari, of course, but then I get all the ads. ;) Any new ideas?