If you want to download from multiple folders, I suggest IMAP.

Note that the limit for POP downloads is 1250 MB per day:

https://support.google.com/a/answer/1071518?hl=en

Thanks for the two replies so far. Regrettably they do not address the primary problem: How do I retrieve the roughly 40,000 messages from 2008 to 2022?

I understand the problem of bandwidth, but two days later, I still cannot retrieve the older messages and I cannot figure out why Thunderbird, apparently at random, started retrieving the messages beginning in December 2022 rather than in 2008.

If I were to choose to use IMAP rather than POP3, would the entire contents of the messages in my GMail inbox be downloaded to my computer?

If you use IMAP, whatever is on the server would be accessible. you would need to use file>offline>download to retrieve them all.

David,

I wonder if I can use two separate Thunderbird profiles, one with POP3 and one with IMAP to retrieve the same messages from my GMail inbox. So far, none of my old messages from before December 23, 2022 have been retrieved.

Best regards,

Eric

You can have both in the same profile.

Hello again David,

Could you please elaborate? I have experience going back to Netscape Communicator aand before but I'm a bit at a loss with the intricacies of Thunderbird.

Best regards,

Eric

Since you already have a POP account, just - click 'account setting' - on left side, below current account info is 'Account Actions drop-down menu. Click and select 'add account' - then fill in name and email id and do NOT enter password and press enter. I think TB will default to IMAP setup