Major issue with Thunderbird and Mcafee

Set McAFee not to scan emails. Then go to TB menu > Settings > Privacy & Security > Security > Antivirus and enable 'Allow antivirus clients to quarantine individual incoming messages'. Or

Too late. I installed BetterBird and it went away.

So you installed Thunderbird with some soft patches and it went away. So nothing changed except the McAfee rubbish (toolbars)did not install in betterbird, or you were the subject of a scam. Do you even have Mcafee installed?

I googled the message you posted

Trojan:Win/SuspiciousAttachmentScript.a" the google AI suggested

Trojan:Win/SuspiciousAttachmentScript.a is a malicious detection by Windows Defender indicating that a script-based threat has been associated with a suspicious file or email attachment on your system.

and offered two link as it's source https://www.bleepingcomputer.com/forums/t/815781/constant-malware-attacks-for-the-last-24-hours/ and https://blog.360totalsecurity.com/en/how-to-remove-trojan-virus/ (I did not look far with this one as it was an antivirus vendor blog.

The next thing I must point out is that Thunderbird does not generate random attachments. nor does it run scripts in emails. I can find no information at all related to a script NMSA. most references I could locate refereed to various national maritime safety associations and authorities although the Naturopathic Medical Student Association did make for a subtle change of pace.

So the implication is that mcafee (I do have it installed) somehow instigated this rogue Trojan. Makes sense because this started almost to the day that my my subscription needed renewing.