Hello,
It is pretty suspicious that you have Flash 15 in Ubuntu (unless I am misunderstanding you) - on Adobe's website (http://get.adobe.com/flashplayer/):NOTE: Adobe Flash Player 11.2 will be the last version to target Linux as a supported platform. Adobe will continue to provide security backports to Flash Player 11.2 for Linux.
![Application Application](/uploads/1/2/6/5/126528053/102770517.png)
Can you check your /usr/lib/mozilla/plugins/ folder to see if you have duplicate Flash plugins there? Another location to check is /usr/lib/adobe-flashplugin/
As for your Thunderbird question, try asking in that forum:
Jul 16, 2019 How to Install Flash Player on Ubuntu. Flash is no longer being developed for Linux, and the newest versions are only available built-in to Chrome. If you use the Chromium browser, you can extract the Flash plugin from Chrome and use it.