Can you post a crash report URL from Help/Troubleshooting Information? Many crashes are due antivirus incompatibility, especially after a TB update, so in the meantime start Windows in safe mode with networking and see if TB works OK. State the mail account provider and your server settings, if you're doing a manual setup.

Thanks for the response. I tried running TB in safe mode with networking & still get the message that it's unable to connect. I tried to send the crash reports , but haven't successfully uploaded them to you. Where is the Help/ troubleshooting information? I think "maybe" I have found the problem though. Even though my Bitdefender program was disabled when I tried to start TB , I found a file that looks suspicious to me in my Thunderbird folder. I emailed Bitdefender Support to see what that file is, but haven't received an answer yet.

See this discussion about Bitdefender:

https://support.mozilla.org/en-US/questions/1275164

I finally discovered my problem logging into my Thunderbird account. The window asking for my password wasn't asking for my Thunderbird password. It was asking for my ISP, Comcast, email password.

So your anti virus has subverted Thunderbird's use of it's own certificate store in favour of the less secure one Microsoft provides. Why, so they can make your communications available for hacking. They say they need to reduce your overall protection and security to protect you. Because they can not scan your encrypted communications without decoding them. I just hope you truly trust those providing your anti virus product. The truly weak link is now any fault that might appear in that product that is updated daily could leave your system completely exposed. But if you feel you can completely trust a company from Romania with your security that will be Ok.

Actually , I thought everything was solved because after using my Comcast email password I can receive incoming email with TB. I discovered today though that I can't send outgoing email. I get this message after the connection fails : Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.comcast.net was lost in the middle of the transaction. Try again.

I have checked with Comcast & the port numbers & type are correct.

I don't think I understand the TB certificate problem. The TB prompt recognized my Comcast password when setting up an existing account. I'm not sure if that's what you were meaning.

Did you follow the advice to adjust Bitdefender's settings? It may be sufficient to add thunderbird.exe as an exclusion in BD, but if it's also scanning outgoing mail, disable that as it is probably the source of your sending issue.

I'm at a complete loss in figuring this out. My Bitdefender has an exclusion for thunderbird.exe & the antivirus is set to only scan incoming email. When I look at my TB settings & click on the SMTP server, I get a message that an account is already set up. I still can't send an email though.

You have two accounts with the same name timkelly... In the Account Name box in your picture, change the top one to something distinct like timkellyIMAP and the other one to timkellyPOP. Don't allow BD to scan incoming or outgoing mail. The real-time background scanner is sufficient, and won't cause the types of problems mentioned in the link in my 2nd reply.

I tried changing one of the accounts to timkellyImap, but I still get a message that an incoming server already exists.

I only want one account, but I'm not seeing how to eliminate the other account altogether. I just want my one account, but have it work for both incoming & outgoing.

When you access the account settings, and click ok to that message the cursor will normally be blinking in the account name field of the problem account and you can type a 1 and click ok to effect the renaming of the account.

However your image shows you somewhere in identities. Why you are there I have no idea. When you click ok does the displayed information change?

I don't know why my install of TB doesn't seem to be working right. I don't see any blinking of my name in the account field. Actually nothing seems to function OK anymore. I'm no longer seeing my incoming emails as an incoming list. The only way I can get to the account settings window is to open a new email to write, then go to tools , then account settings. Once I'm there though , the only thing I'm able to see is the identities window. How do I eliminate one of my accounts so I only have one account?

Accounts can be removed from Account Actions in Account Settings (bottom left). In your case, the IMAP account is Default and can't be removed unless you set the other account as Default. If for some reason you can't remove one of the accounts and get it working, you could create a new profile from Help/Troubleshooting Information, about:profiles, then add an account to the new profile. The old profile will be unaffected.

I think my extra account has been removed OK now. I can't see my list of emails though. There seems to be only a few Thunderbird pages I can navigate to. How can I get back to my inbox?

I guess this is my inbox, but where are all my emails?

Press Alt to show the Menu Bar, then enable View/Layout/Folder Pane. Also, deselect the buttons on the Quick Filter Bar (Unread, Contact, Tags).

That worked to get my emails & folders back!! Thanks!! I still am having the problem of not being able to send outgoing email though. The server port # have been verified with Comcast.

This is the message I get when trying to send an email:

Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.comcast.net was lost in the middle of the transaction. Try again.

My Bitdefender firewall is set to allow traffic both ways for TB & the antivirus is set to scan only incoming email, but I still can't send email with TB.

What do you have for the Comcast outgoing server? It should be smtp.comcast.net on port 465, SSL/TLS security, authentication = normal password, User Name = email address.

https://www.xfinity.com/support/articles/setting-up-thunderbird