Trying to import an MBOX file to gmail account in Thunderbird

I was able to import an MBOX file to a local folder. When I try to import it to a folder in my gmail account, I receive the message “Impossibile to import in this folder (imap or newsgroup type)” Does anyone know how to fix this?

Import to the local folder, then copy or move the messages to the IMAP account.

Thanks, I tried that, but it stops after 235 messages and I have a couple hundred thousand that I need to move.

Mail servers are not designed for mass transactions, and most email providers limit the no. of possible transactions within a certain time interval.
So you’ll have to upload your messages in batches, and not all at once.

Not true.

I’ve done many migrations to and from one platform to another using ImapSync, with some accounts having many many GBs (and hundreds of thousands of emails) of emails.

But, I have always had issues trying to do mass moves/copies when using Thunderbird or any other client.

This is almost certainly a client-side issue.