A new release of Gnash, version 0.8.9, is due in the near future. Plenty of bugs have been fixed, but some users are still going to have problems playing YouTube videos. Here's a short explanation.
At some point last year, YouTube started setting HTTP cookies in your web browser, to keep track of which of their video servers is nearest to your machine. This lets them provide a better experience for you (I guess). Here's a diagram of what goes on in your browser:
As of Gnash 0.8.8, a workaround was added, calling an NPAPI function NPN_GetValueForURL to ask the web browser for the relevant cookies. New diagram:
The downside is that this function was only added in NPAPI version 1.9. This means:
What does Adobe's plugin do? I imagine it's something like the following:
So, the good news is, this problem should almost certainly get fixed over the coming Debian release cycle, if only because all the browsers will get updated. In the meantime, disabling cookies (and removing stale ones from YouTube) is a workaround.