Office360 and OAuth2 not working with IMAP

[AEtherScythe]

My company is in the process of switching to Exchange on outlook.office365.com.
I have been using our beloved SeaMonkey with our old e-mail infrastructure and Mozilla since before SeaMonkey.
Unfortunately, I cannot get SeaMonkey to work with our SSO-enabled / OAuth2 authentication scheme on Office365 (via company login).

I referenced this thread:
viewtopic.php?f=3&t=3060515

And this RFE:
https://bugzilla.mozilla.org/show_bug.cgi?id=1293958

I read elsewhere, that SeaMonkey 2.53.x picked up the above RFE, but it isn't working with Exchange on outlook.office365.com.
When I try to use OAuth2 in SeaMonkey, I get a message that outlook.office365.com doesn't work with that authentication scheme.
When I try to use normal (basic) auth, with my password it doesn't work (not surprisingly, since my company insists on SSO via OAuth2).

I even went so far as to install Thunderbird 78.6.0, to confirm that the OAuth2 support works with my company's SSO handling and it does.
So I took that working profile from Thunderbird and copied it to SeaMonkey 2.53.5.1, and SeaMonkey understand the profile perfectly well, but the error message is the same:

"The IMAP server outlook.office365.com does not support the selected authentication method. Please change the 'Authentication method' in the 'Account Settings | Server settings'."

I even went so far as to access the https://outlook.office.com/mail/inbox webmail via SeaMonkey browser, and I authenticated all the way through to Outlook and told SeaMonkey to remember me / save my credentials, but unfortunately that was no help.

I was really hoping that all that was done re: authenticating to https://outlook.office.com/mail/inbox would be stored in the way of cookies and such, so that the SeaMonkey mail client could utilize the same stored credentials for IMAP, but it isn't working that way.

Any advice?
Perhaps some hack to take the outlook.office.com credentials from the browser and copy them to outlook.office365.com IMAP credentials in the profile (and/or cookies)?

Meanwhile, please advise how I can get further diag / debug logging on MacOS to aid in uncovering what exactly isn't working with the OAuth2 / SSO to outlook.office365.com.

[AEtherScythe]

I referenced @frg's comment on a similar topic over here: viewtopic.php?f=3&t=3055147
Hoping @frg or another will have some insights on my post above. :S

[frg]

This needs a bunch of backports. Was only fixed for Thunderbird 77. The first batch will be in todays unofficial 2.53.7b1 pre build. If I manage I will do the remaining parts next but need to make sure caldav access is not affected.

In any case I can't test because I am staying away from google and ms stuff as far as I can So please test todays builds for regressions using OAuth2. It already added support for yandex.com and using pop3 with OAuth2. Might work might not so keep the old one. Bill tested google mail briefly.

FRG

[frg]

The backports now landed. Prerelease builds are available at:

https://www.wg9s.com/comm-253/

Unable to test. gmail still works but no one from the dev team has an office365 account. So please test and let us know. While it is prerelease code it is very stable and should cause not much problems. Still profile compatible with 2.53.5.1 but Lightning needs to be reinstalled if for whatever reason you want to go back.

[Rob_S]

Working fine for me as browser and email client, but I'm just using standard pop mail accounts with Rockland.web.