Clear cache and cookies (for that site) and try again.

Well, that was a simple one. After my comically large cache has gone (freezing Firefox in the process) the videos work as expected. Thanks!

Actually, no. It works when I tested on a music video, but doesn't work in a channel. Odd...

There may be more than one cookie for the site.

I got rid of everything returned by searching for 'youtube' in 'remove individual cookies' - could they not be searchable by this?

If it is in fact the "YouTube" site then there will be a cookie folder on the left bearing that name and there will be a + sign next to it. Clicking the + will expand the folder revealing all of the cookies relative to YouTube. And yes, YouTube is searchable as you indicated. However, removing just cookies labled YouTube may not remove all related cookies if there is another server being accessed within the YouTube server farm,etc.

OK, I deleted the folder called 'youtube.com' for good measure and that hasn't solved it. I also enabled my 'HD Suite' scripty addon thing on a whim, and that has the same problem - it can't auto-play the video in a channel at the highest quality, but can do it for a normal video...

Have you updated your video drivers? And I assume you have an HD capable video card? If so have you enabled HD or reset the preferences back to default?

I don't know what you mean by enable HD, but I'm fairly sure my reply should be in the affimative. I updated my drivers a couple of hours ago (not to fix this, just because I hadn't done it in a couple of months) for my Radeon 5770 and it plays HD content - videos and games, anyway - really quite well.

This video http://www.youtube.com/watch?v=ZFHxtnacFV8 automatically changes to 720p in fullscreen, and works fine. When played in the channel window @ http://www.youtube.com/user/delphicmusic it only automatically changes into 480p in fullscreen.

I just tried the delphiemusic link and it started in 480p but when I switched to full screen and then back it was in 1080p - mystery, don't know but it appears to not be a Firefox issue.

Mystery indeed. And irritating in a really minor way. Thanks very much for your time, though.

Just tried it again and it's behaving as intended. It starts in 480p and using the drop-down I changed it to 1080p, switched to full-screen and still in 1080p and back to normal screen, still same 1080p. Also the same using the old Navigator 9 that I keep around for just this sort of testing.

Yeah, mine'll maintain resolution once I set it. Apparently having high quality video presented to me on demand and within a few seconds just isn't enough for me. I want it to be automatically unreasonably high quality without having to press a button to say so. Kids today...

I just tested in Chrome again, and it has the same issue. I must've only tested a non-channel video earlier. Bit of a waste of both our time, this. Over to Youtube support I suppose. Woops. Sorry.