Due to recent problems on YouTube, I decided to create this thread.
If you are experiencing flash problems only on YouTube in fullscreen and instead of video you get only audio, then you need to install Flash 11 Beta (you can do that with using Flash-Aid) or disable hardware acceleration. You can do that by right-clicking on a video, selecting "Settings", then the "Display" tab. If you are using Flash-Aid and don't want to use Flash 11 Beta, you can alternatively untick the option to Override GPU validation in the tweaking options step.IMPORTANT EDIT: Just updated Flash-Aid to get the new Flash 11 Betas, for both 32bit and 64bit. If you already use Flash-Aid, all you need is to execute the Wizard to install. Hardware acceleration and the performance tweak "Override GPU validation" is working on YouTube again. No more black screen
If YouTube videos doesn't work on the main site, but works embedded on other sites, then delete the YT cookies, clear the cache, then block YT cookies in Firefox Privacy preferences - you need to select "Use custom settings for History", then click the "Exceptions" button, then add a "Block" rule for youtube.com
If that doesn't work, try https:///www.youtube.com