Thunderbird 91, a new major version of the open source cross platform email and communication client, has been released.
www.ghacks.net
Not too hard to find, but PDF.js ( same viewer that's used by default in Firefox ) was integrated in Thunderbird version 91 released August 2021, and wow that's a pretty busy bee of developers on Github.

Simply keeping Thunderbird updated, I have a hard time see where this actually could be dangerous enough and extra since I personal as a malware hunter very rarely stumbles over actual alive and still kicking in the wild genuine malicious pdf samples. The majority of those are phishing attacks. Phishing links normally have a very short time span where they are still active. Even a CVE search gives very little info:
Pdf.js Viewer Project Pdf.js Viewer security vulnerabilities, exploits, metasploit modules, vulnerability statistics and list of versions
www.cvedetails.com
but click around too much anywhere on pdf files, is bound to ask for trouble no matter what OS, browser, software etc as it's delusional at best claim something is 100% bulletproof/secure.