(Well, this is several hours after posting and it still didn't show up, so trying again.)

First off Stans, thanks for the reply. Much appreciated!

Good to know that the Firefox install is completely separate from the TB setup. That at least eliminates one variable from the troubleshooting equation.

I updated TB to 115.1.1 and tried again but unfortunately still "no go". First attempt told me that cookies were being blocked and need to be allowed. I changed my cookie setting from Custom to Standard, refreshing all tabs. Still the same problem (I tried with and without providing the password in the initial configuration window). I also tried clearing the cookies, data and cache and retrying but that made no difference.

Just to quickly restate what is happening when I follow the instructions in the link provided above, I get that window that opens ("Enter credentials for <my email> on outlook.office365.com") and in the "Setup your existing email address" window it says "Checking password..." (right above "Available configurations") and at the VERY bottom of the window it states "Mail for <my email>@outlook.office365.com: sending login information ..."

After a lengthy wait with nothing happening, I click Cancel and get the following message in a yellow box (where the "Checking password ... " message was):

"Configuration could not be verified. If you username and password are correct, it is likely that the server administrator has disabled the selected configuration for your account. Try selecting another protocol."

I have clicked on the Manual Configuration button just to review the settings and confirm they are as per the linked document. They are:

Incoming Server: IMAP outlook.office365.com 993 SSL/TLS Oauth2

Outgoing Server:

smtp.office365.com 587 STARTTLS Oauth2

Is there something about the linked document / these settings that is no longer correct? Do I need to configure differently?

Also, I have several accounts under my TB app; this GoDaddy/exchange account being one of them. It is an email I use for a business (very small "b" but business nonetheless) and does have emails I wish to keep. Is the fact that this email account is one of the pre-existing accounts within TB somehow prevent this Set Up Another Account function from working? Last year GoDaddy forced an account change upon its users and in that situation I was able to create the new account for the same email account, drag my folders and inbox emails over to the new account and then delete the previous account.

Thanks in advance to any and all that can assist with this perplexing and most frustrating problem.

Second attempt also failed.

Just saw this at the very top:

"A moderator must manually approve your post before it will be visible."

Is this standard M.O. or is this indicative of a word in my post that has been (mis)interpreted as offensive by some kind of software?

Second attempt also failed. Just saw this at the very top: "A moderator must manually approve your post before it will be visible." Is this standard M.O. or is this indicative of a word in my post that has been (mis)interpreted as offensive by some kind of software?

Any replies that have links that are not whitelisted are auto-flagged needing moderator approval. This forum gets occasional spammers and while spam in OP can be easy to spot, the spam in replies may get missed if not for the auto-flag system put in use. You now have multiple duplicate posts in this thread as you kept trying.

Remove that account from Thunderbird, Quit then relaunch TB and try to add it again. Leave the password field blank because TB won't be using the password for Oauth2 authentication. The Oauth2 pop up should show up. If the MS 365 sign in page still loads partially, then something else could be interfering with TB's Internet access. Security programs (e.g antivirus software) are notorious for this. So, which security program do you have running?

Is IMAP access still enabled for this account? Knowing Microsoft, their default security policy disables (legacy protocols) IMAP/POP and SMTP in favour of their Exchange protocol. You have to manually re-enable those legacy protocols to access using a client that doesn't support Exchange (such as TB).

You can narrow this down to whether it affects TB only, by setting up that account for IMAP access on the Windows Mail app. If the Oauth2 process goes through without any problems, we can try something else in TB.

James, thank you for the clear explanation. In the future I will put something like

outlook dot office365 dot com

to avoid this type of delay.

Thanks again for your help Stans. If I remove the account from TB, I would lose that account's email (which is important to me), would I not? If yes, can I set up a folder on one of my other TB email accounts and move this account's folders under that folder, drop the account, recreate the account and then move all those folders back?

My security programs are MalwareBytes and SuperAntiSpyWare.

How do I tell if IMAP access is still enabled for this account? If it isn't, how do I re-enable it?

Thanks

Update:

I have IMAP working. As mentioned above,

"First attempt told me that cookies were being blocked and need to be allowed. I changed my cookie setting from Custom to Standard, refreshing all tabs. Still the same problem (I tried with and without providing the password in the initial configuration window). I also tried clearing the cookies, data and cache and retrying but that made no difference."

It finally dawned on me that this has nothing to do with Firefox cookies as it has nothing to do with Firefox (as somebody mentioned above). The "browser" setting is for Thunderbird's "browser"! For anybody in the future that may come across this thread, do the following:

Tools -> Settings -> Privacy & Security. On this page, under the Web Content section, check the "Accept cookies from sites" checkbox.

I was able to create a new IMAP account (as per the link somebody provided early in the thread) with the same name as the POP account. Now I am just migrating folders and emails over to the new account and will then remove the old account.

Thanks for the help everybody.