Youtube – ny way to disable DASH playback in the HTML5 player

I'm not sure about Flash, but I recognize the problem you're having. Firefox's <video> support hasn't yet been done properly, so it's much slower than any other method.

If nobody else suggests something you like better, FlashVideoReplacer would let you swap out the YouTube-provided player for either the Quicktime or VLC plugin and both should run faster than Flash or Firefox's internal <video> support.

(It also supports various players which appear in their own windows, but you probably don't want that)

Another option you could try would be to track down WHY YouTube is forcing HTML5 on Firefox by trying these one-by-one:

• deleting all YouTube cookies (YouTube's fault but configurable)
• running in Safe Mode to disable extensions (One of your extensions' fault)
• using the User Agent Switcher extension (YouTube's fault, not intended to be configurable)

Finally, if none of that works, you could give Google Chrome a try. Its extension system isn't as powerful as Firefox's, but AdBlock Plus does have a version for it.

Some videos on YouTube are uploaded in a high resolution, for example 720p (720 pixels vertical resolution). However, the player window itself is just around 380 pixels high.

When you watch the video in the small player, it doesn't make sense to buffer the high-quality version. You wouldn't see all the pixels anyway.

When you switch to full-screen, notice that the resolution indicator in the bottom right can change, for example, to 480p or 720p, which is a completely different stream. Therefore, it has to be re-buffered.

You can change this behavior under Settings » Playback Setup » Video Playback Quality: