Taking the right actions to ensure that all your messages are in Thunderbird (e.g., set for off-line use) may be sufficient (I don't know), but then Thunderbird would keep trying to connect to that account after it was deleted, so you would want to move the messages to local folders later anyway.

The best thing to do is to to copy all the server folders to your local folders account. You can even select the folders and copy them all in one step to the local folders account. Very easy.

Back up your profile first.

You'll have to decide when to do that and how to monitor for new messages that must be copied to local folders too. Or you could create a message filter that moves all new messages to a local folder so you don't have to monitor.

When you're done, back up your profile again.

I know IMAP pretty well. I don't know much about gmail. One thing to consider with gmail is the allmail folder. You'll want to make sure that you have a good understanding of what is in it before doing anything with it.

Thank you Rick for your time and input!

Just to be sure we are talking about the same things.

When you say "copy all the server folders to your local folders account" you mean download all emails to my hard disk?

When you say "copy all the server folders to your local folders account" you mean download all emails to my hard disk?

No. Thunderbird has an account called "local folders" for storing messages only on the computer. Other accounts (server accounts, e.g., gmail) store messages both on a server and on the computer. IMAP keeps them in sync. You want 1) to ensure that all the messages in your server account are downloaded from the server to the computer, then 2) copy them to folders in the local folders account. Then those messages wil be on the computer no matter what happens to the server account (gmail).

To do 1, which you asked about: Go to account settings > synchronization & storage and check "Keep messages in all folders for this account..." Then click "Advanced" and select all folders. I don't know if that second step is redundant.

This next step may also be redundant, but I would check the settings just to be sure: For each folder with messages that you want to preserve, right click on the folder, select "Properties", go to the "General information" tab and check "when getting messages for this account ...", then to go the "Synchronization" tab, and check "Select this folder for offline use", then "download now".

To do 2: After that, copying folders from a server account to the local folders account is easy. All the folders that you want can even be copied in one step. You can shift-click or control-click or command-click to select multilple folders. Then right click on them, select "copy to", and copy them to the local folders account. Copying is quick. I tested this process earlier today and copied 2,000 messages in a few seconds.

You should back up your profile before you do anything and after you do everything.

If you copy folders now while the gmail account is still active, you will have to decide what to do about new messages arriving in your gmail account that are not in your local folders. One approach would be to create a message filter to copy them automatically to a local folder. Then your local folders will be current at all times. Oh, you would also have to create a filter to copy sent messages. Or you could wait until right before the gmail account is disabled before copying your folders to the local folders account.

Allmail: you may know that it holds all messages, including those in trash. Your messages have other gmail labels that show up in Thunderbird as folders. If you subscribe to allmail, you will have duplicates of all your messages. A common recommendation ion the forum is not to subscribe to allmail in Thunderbird because of the duplicates.

I don't use gmail and I am not an expert on it, but I believe that those statements are true and helpful.

Keep asking.

Super helpful! Thank you Rick!

I cannot copy “Inbox” to “local folders”. Any hint to make that happen?

Plus: I am still unsure whether or not all emails got already copied downloaded. How to ensure it before copy the folders from Gmail acc to local folders acc?

I cannot copy “Inbox” to “local folders”. Any hint to make that happen?

Create a new folder in Local Folders for the inbox (named something else), select all the messages in your gmail inbox, right click, and copy them to that new folder.

I am still unsure whether or not all emails got already copied downloaded. How to ensure it before copy the folders from Gmail acc to local folders acc?

Monitor the status bar at the bottom of the window. If you don't see it, View menu > Toolbars > Status bar. The activity manager helps too. Tools > Activity manager. If you have set all the settings that I suggested, and you have gotten all new messages (File menu or button in the folder pane header, and the status bar is not reporting activity, and the activity manager reports that the account is up to date, you can probably feel confident that you got all the messages.