for OAuth2 to work smoothly in TB , Cookies need to be enabled in TB, bit more info here.

you may also see Microsoft's Exchange ActiveSync , if you also use that to access mail, here.

I tried to disable cookies and adding exceptions for the URLs mentioned in that post, but now when I microsoft's authentication window pops up, it doesn't even validate my password.

So I assume that O365 are not officially supported in Thunderbird yet, right?

If your mail-server is using MS Exchange-ActiveSync protocol/server , then TB = Thunderbird (an email client software) cannot connect with that MS Exchange mail-server , unless the (TbSync) Addon/Extension mentioned inside the earlier link is used , even after that obviously all activesync features/functions are not supported.

there are also various server-tools, that can be used with opensource & non-Microsoft software based mail-server software+tools , to provide support for (some of the) activesync compatible services to email-client/users.

So if your mail-server is MS-Exchange based OR if your mail-servers do not have specific activesync-compatible tools with open-source mail server , then TB email client software will not work.

"office365.com" based mail-servers can work in TB , if ActiveSync option is not used & not enabled for the Microsoft email account.

you said you've disabled cookie, but, Cookies option in TB, needs to be enabled for OAuth2 "auth-method". enable All Cookies for now (for Microsoft) , as we still dont have specific Cookie-Exception URL list for Microsoft mail-services. TB main-menu > Options/Preferences/settings > Privacy > Web-Content > select/enable (ALL) "Accept Cookies From Sites".

when Cookie-Exception URL list will be available for MS, then disable "Accept Cookies From Sites" , and add+allow that list of URLs inside the (Cookie)-"Exceptions" list in TB . When this list will be available then it will be added first in the bottom-side-last link i mentioned at bottom side of this.

sometime in TB a mail-account can keep on asking for password , when incorrect protocol are selected or when earlier/older password is not removed first, before adding a new password or secure-mail-key or when a setting was changed, etc, etc. so please remove earlier password for that mail-account in TB: TB main-menu > Options/Preferences/settings > Security > Passwords > Saved Passwords > click on "Username" column's header to change sorting type to find accounts quickly > find your specific mail-account > select it > Remove.

And please also wait for other SUPPORTERS to also assist you on these, ( one person cannot know ALL or have solution for everything )

when OAuth verifying browser-tab comes for MS mail-account in TB , then try to take screenshot(s) by pressing specific combination of keys in your OS , whenever URL/site changes in that tab , keep on doing these until the auth completes, after above, if you create a list of URL of those sites from screentshot pictures, without the ..... letters/codes after the .com/...... part , then that is the list of MS Exceptions needed in "Exceptions" list in TB.

Thank you atEric.

To be honest, I don't know if the Activesync option is enabled or not. It's just my work email and I have no idea about its configuration. Just that it uses two-factor authentication to log in, and it goes through Office 365.

I'll see if anyone else has had some experience in these cases.

remove earlier account , & remove SAVED-PASSWORDS , restart TB. please load Addon(s) mentioned in the top-side link, in your TB. ( ok, i'm including one addon here: TbSync, etc addon. ) after adding those addons/extensions, you should restart TB again. then add a new MS Exchange ActiveSync based mail-account in TB, & try to connect with your office mail-server, see if that works or not.

TbSync has nothing to do with mail. It's for calendars and contacts only.

https://support.mozilla.org/en-US/questions/1305803

https://bugzilla.mozilla.org/show_bug.cgi?id=1668834

Try this ExQuilla addon or get from here.

oops, forgot to say : with TbSync, also use ExQuilla. i assumed the link which i have been mentioning in top/first post, you've already read/seen it , it has ExQuilla addon, etc, etc.

Oh, I heard about ExQuilla before, but it's a paid addon... : (

or use DavMail thats free , its in the top link. DavMail creates a local gateway inside your own computer, TB can use it.

for Linux Ubuntu , there is BlueMail email-client app. BlueMail for other platforms, here.