You have to create a new account in Thunderbird, and specify that it is for IMAP. Once an account in Thunderbird is created, you cannot switch it from POP to IMAP, or visa versa.

From the "three bar" menu button on the top right, click: New message: Existing Mail Account

Then use the settings you see in the screenshot below.

Thank you, Bruce, but I'm afraid I might have created another problem. I now receive emails addressed to the new gmail account. But the column in THUNDERBIRD to the left shows two users each with the same email address for the gmail account. There is also a "Local Folders" account showing beneath those two. The first of the gmail accounts shows all the Folders I had been using previously with the verizon account. The second gmail account has the standard basic folders, such as Drafts, Sent Mail etc. and the one new FOLDER I'd created in gmail, which means it was imported to THUNDERBIRD from my Google Gmail. When I try to open the ACCOUNT SETTINGS windows for either of the gmail accounts, this window appears: "An account with this name already exists. Please enter a different account name." If I try to send an email from either account, I have the original problem of my password not being accepted. Thank you.

The problem is that you put those gmail settings in the old verizon account.

I recommend moving all the e-mails and folders you created in the old account, to Local Folders to save them. Then remove the old account. Do not remove the old account without moving those e-mails, or you will lose them.

I do not recommend moving the e-mails and folders from the old account to your Gmail account. Uploading lots of e-mails this way to an IMAP server usually causes problems with messages going missing. IMAP was never designed to work this way.

Once you have moved all those e-mails to the Local Folders, then:

Right-click that old account in the normal folder view of Thunderbird and choose Settings.

Click the Account Actions button on the bottom left and click on Remove Account.

Bruce, I think we've made some real progress. I carefully followed all your instructions to delete the old accounts after I had first transferred their contents to the Local Folders.

Now, when I open SETTINGS for Local Folders, I see this message: "The following is a special account. There are no identities associated with it. Account Name: Local Folders Local directory: /Users/rmarkman/Library/Thunderbird/Profiles/cjnbcno0.MARK/Mail/Local Folders"

Please tell me what I need to do next in order to create the settings for an account with my identity.

Once again, THANK YOU VERY MUCH. I really appreciate your taking the time to walk me through this, and responding so quickly. I wish I could reciprocate in some way. . . ???

I said to remove only the old account, but you have removed all the accounts, and then tried to remove Local Folders, which is not allowed. It's a good thing that you cannot remove Local Folders, because if you followed my instructions, all your old e-mails are in Local Folders.

At this point, you need to repeat what I said to do in my first reply, which is to create an account in Thunderbird for Gmail using IMAP.

Okay, Bruce. Sorry for not following your earlier directions correctly. I've created the new account now. But it looks a little strange and I wonder if this is right. Let me explain. At the top of the left column an account identified with my gmail address appears. When I click on the triangle next to it, "INBOX" will appear right underneath. And directly below INBOX, but not indented, is a blue folder icon with "[Gmail]" next to it. That word is greyed out. When I click on the triangle next to the Gmail icon, a long list of folders appears. I've created these to sort my emails. If I open any one of those folders, it takes a while for the emails for that folder to appear. They have to be loaded. This seems odd to me because on my previous Thunderbird, for Verizon, the emails in a folder appeared instantly. There was never a visible loading and a spinning blue wheel icon. Another strange thing now is that sometimes when I open Thunderbird none of the appear. All that is there is my new gmail account address, but nothing beneath it, not even "Inbox." If that happens I close Thunderbird, and reopen it. Usually then all the folders for the new gmail account will appear. One last oddity is that sometimes now when I open a gmail subfolder, and the emails are loading, an error message window opens at the top of the Thunderbird screen saying that the connection to the server has been lost. Have I done something wrong again?

... At the top of the left column an account identified with my gmail address appears. When I click on the triangle next to it, "INBOX" will appear right underneath. And directly below INBOX, but not indented, is a blue folder icon with "[Gmail]" next to it. That word is greyed out. When I click on the triangle next to the Gmail icon, a long list of folders appears. I've created these to sort my emails.

This is all normal so far.

If I open any one of those folders, it takes a while for the emails for that folder to appear. They have to be loaded. This seems odd to me because on my previous Thunderbird, for Verizon, the emails in a folder appeared instantly. There was never a visible loading and a spinning blue wheel icon.

The first time looking at the folders, Thunderbird is listing the e-mails and downloading them (synchronizing) to your computer for the first time. This will take time. The next time you go to the same folder, it will be quick.

Another strange thing now is that sometimes when I open Thunderbird none of the appear. All that is there is my new gmail account address, but nothing beneath it, not even "Inbox." If that happens I close Thunderbird, and reopen it. Usually then all the folders for the new gmail account will appear.

This can happen if there are problems with the network connection to Gmail. Most likely your Internet connection.

One last oddity is that sometimes now when I open a gmail subfolder, and the emails are loading, an error message window opens at the top of the Thunderbird screen saying that the connection to the server has been lost.

Again, this is most likely a problem with the network connection to Gmail.

THANKS, Bruce. I don't understand why I'd be losing the internet connection. It doesn't happen for any other apps or services and never used to happen on THUNDERBIRD. The folders seem to have to be repopulated every time I reopen THUNDERBIRD. Maybe I will try to remove the GMAIL account and then add it again again. What do you think?

Same problem - renders Thunderbird unusable - turned on 2 step verification - no help.

I am still in email setup - still refuses to accept password.