YouTube video tells me to install Adobe Flash with Gnash installed

I don't think this is a Trisquel bug. That is a message from youtube that we can't control.

IMO it is like if you visit a website and it says "you should install x non-free software". That is a problem with the information being published on the website but isn't a problem with the browser.

Edit: Or is the bug that the video simply won't play? Maybe for some reason that video requires a higher version of flash (gnash) then most normal videos.

You must read this: https://trisquel.info/en/wiki/play-videos-without-using-flash

That video is not in WebM format. Appending "&html5=1" to the URL does nothing.

And it doesn't work with http://packages.trisquel.info/brigantia/youtube-dl (before using it, run $ sudo youtube-dl --update) (Also, I use alias youtube-dl='youtube-dl --prefer-free-formats' in my ~/.bashrc):

$ youtube-dl --list-formats https://www.youtube.com/watch?v=8Esu3DJ-eGU
[youtube] Setting language
[youtube] 8Esu3DJ-eGU: Downloading video webpage
[youtube] 8Esu3DJ-eGU: Downloading video info webpage
[youtube] 8Esu3DJ-eGU: Extracting video information
[youtube] RTMP download detected
[download] Destination: 8Esu3DJ-eGU.flv
ERROR: RTMP download detected but "rtmpdump" could not be run

$ sudo apt-get install rtmpdump
[...]

$ youtube-dl --list-formats https://www.youtube.com/watch?v=8Esu3DJ-eGU
[youtube] Setting language
[youtube] 8Esu3DJ-eGU: Downloading video webpage
[youtube] 8Esu3DJ-eGU: Downloading video info webpage
[youtube] 8Esu3DJ-eGU: Extracting video information
[youtube] RTMP download detected
[download] Destination: 8Esu3DJ-eGU.flv

ERROR: unable to download video

On second reading though gnash will play most youtube videos that are not webM. I think s/he might be reporting the bug as gnash won't play that particular video.

Correct! That's what I was reporting.

Thank you for taking the time to report this bug and helping to make Trisquel better. The issue you are reporting is an upstream one and it would be nice if somebody having it could send the bug to the developers of the software at https://savannah.gnu.org/bugs/?group=gnash. If you have done so, please tell us the number of the upstream bug (or the link). Thanks in advance.

Reported upstream:

https://savannah.gnu.org/bugs/index.php?36724

Marking as duplicate. Upstream marked as duplicate of other bug. See:

Trisquel issue #5831 (Gnash will not play Youtube videos?)