The remote Ubuntu 20.10 host has packages installed that are affected by multiple vulnerabilities as referenced in the USN-4736-1 advisory.

  - When a HTTPS pages was embedded in a HTTP page, and there was a service worker registered for the former,     the service worker could have intercepted the request for the secure page despite the iframe not being a     secure context due to the (insecure) framing. This vulnerability affects Firefox < 84. (CVE-2020-26976)

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

Ubuntu 20.10 : Thunderbird vulnerabilities (USN-4736-1)

high Nessus Plugin ID 146538

Version 1.5