Thunderbird is an email client. It downloaded the message with the link in it from your providers server. It's job is done. It has nothing to do with playing YouTube videos.

Thunderbird is more than just an email client.

I use ThunderBrowse 3.82, so when clicking on links they open up web pages in adjacent tabs.

Youtube videos used to play in these tabs, but they are no longer.

Hello,

To find the correct solution to your problem, we need some more non-personal information from you. Please do the following:

   Use ONE of these methods to open the Firefox Troubleshooting Information page:
       Click the menu button New Fx Menu , click on help Help-29 and select Troubleshooting Information.
       Type about:support into the Firefox address bar and press the enter key. 
   At the top of the Troubleshooting Information page that comes up, you should see a button that says "Copy text to clipboard". Click it.
   Now, go back to your forum post, right-click in the reply box and select Paste from the context menu (or else click inside the reply box and press the Ctrl+V keys) to paste all the information you copied into the forum post. 

If you need further information about the Troubleshooting information page, please read the article Use the Troubleshooting Information page to help fix Firefox issues.

Thanks in advance for your help!

OK, but will this help with the Thunderbird issue, not Firefox where videos play fine.

It looks like I may have solved this.

Went to: http://thunderbrowse.com/faq.php and tried their suggestions at modifying the about:config. Though that didn't seem to help, I turned on anything else I could find related to flash in the config and now it seems to be working.

If anyone is interested I will copy and past the config file if that is possible to do so changes might be seen.

Maybe next time say you are using Thunderbrowse and not Thunderbird. That might help.

The problem is not solved. Though videos are now playing, it is sporadic, often not playing even after multiple reloads, though the 'preview' windows on the timeline show images.

Yes, noting Thunderbrowse right away would have helped, but I forgot that it was not integral to Thunderbird as I had not touched the settings/add-ons in quite a while.