Checking my update histories, my last Thunderbird update prior to yesterday was October 8.
Looking at UptoDown version histories, that would be for 115.3.1
I noticed no changes at that time- hence, I cannot blame 115.3.1
This week on October 18 I got a huge Windows 10 cummulative update.
After that I noticed visual differences in Thunderbird and accounts not working (Gmail account taking hours to receive, Hotmail accounts able to send but not copy to sent folders, Outlook accounts not working at all- asking for cookies) Re entering account passwords for Hotmail accounts didn't work. Gmail didn't need re-entry. Outlook was off the wall.
I updated to 115.3.3 (automatically through Thunderbird) early October 21 to help solve the problem and it made it worse. The "About" tab said it was 32 bit!. The Whole program very slow and even Gmail not working.
I try to install a 102 version of Thunderbird from Uptodown but I thought it wouldn't let me.
I restarted Thunderbird and saw that it had in fact installed 102 version but it had a warning that if I went through with it, I would lose all my data. So I didn't go through with it.
I installed 115.2.0 but saw that it was in Spanish.
I installed 115.3.1 and it worked! Gmail working again. "About" says 64 bit. Hotmail accounts working after entering passwords! Only remaining issue was my Outlook accounts- they were now using Office365 connections. I changed those connections to regular ones that I use for Hotmail accounts eg imap-mail.outlook.com instead of outlook.office365.com and smtp-mail.outlook.com instead of smtp.office365.com
and most important- I went to Thunderbird setting and turned off Automatic Updates.