User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:84.0) Gecko/20100101 Firefox/84.0

Steps to reproduce:

The following is a summary of the experience including the specific script failures and more.
I inadvertently deleted all of my 2020 mail from the inbox. At the end of each year, mail for that year is put in to a folder dated for that year.

The mail was still on the gmail server so it was necessary to download the previous mail. Visible to me on the server was mail for 2019 and 2020. However, after the configuration was changed to get the old mail, mail from 2012 began downloading. Gmail restricts mail to blocks of 25-250 messages at a time. Thunderbird was configured to check for mail every one minute. The process took about 48 hours.

Along the way, I did not keep a log, various problems occurred when I attempted to delete 97,000 old messages. Some issues occurred while trying to move messages occurred. My general conclusion is that TB has issues when dealing with more than approximately 30,000 messages at a time.

During this process, Thunderbird was offline.

Thunderbird Not Responding flashed frequently.
Moving 1434 of 97810 messages to Trash

Unresponsive script
Script: chrome://global/content/customeElement.js:713
Script: chrome://messenger/content/messenger.xhtml:1
Script: chrome://messenger/content/folderDisplay.js:2217

Impossible situation captured with screenshots:
4294967287 messages downloaded
4294967293 messages downloaded

After a lot of work, the mail was restored properly and it is working as expected.

A couple of crashes occurred along the way and reports were submitted.

Had I made a backup prior to beginning, I could have avoided problems.

It would be helpful if there was an option to delete duplicate messages. The add-ons are not compatible.

Actual results:

Script failure as described above. There were some other anomalies listed above.

Expected results:

None of the script failures, crashes and bogus number of messages downloaded.

TB has issues when dealing with more than approximately 30,000 messages at a time.

There are plenty of bug reports on this issue https://mzl.la/38NVwXS - Bug 693659 is the prime one. It may ultimately turn out to be imap related.

Thunderbird is using POP mail.