What is your account type - POP or IMAP?

1) At this point, everything was fine. Thunderbird opened and went to my comcast server and began retrieving emails.

Are there any new messages to retrieve in the first place after that?

Hi,

POP (Under "Account Settings"/ "Server Settings"/"Server Type" it says "POP Mail Server")

Yes, there were new messages to retrieve (about a weeks worth as that's how long the computer was off while the new drive was installed)

With POP the default setting is to remove messages from the server unless you deliberately set the option to leave messages on the server. In other words, you may have removed all messages on the server in your attempt to verify Thunderbird was working correctly.

No, the settings are such that messages are left on the server. --This is how I've always set my mail settings and just verified they are still set this way. --There are currently many messages on the server that are not being retrieved. Also, when clicking "Get Messages", nothing happens ie I wouldn't get the "there are no new messages" message or any other indication that anything was working. As mentioned above, right now with Avast reinstalled, I get the same results as mentioned in (2) above, namely, I get a "Add Security Exception" box.

And BTW, when I click "Get Messages" and get a "Add Security Exception" box and then click on "Confirm Security Exception" in the "Add Security Exception" box, again nothing happens ....no reponse, no error message, no indication that anything has changed.

You know when I first installed Thunderbird as mentioned in (1) above, it did retrieve messages and was working (and this was with the Avast installed). I'd be happy to just somehow get back to this point but when I uninstall Thunderbird and then reinstall, it doesn't work the way it did in (1)

Is there a way to do a "clean" install of Thunderbird? By "clean" I mean the way it installed the first time I downloaded it before I'd copied my back up profile into the Thunderbird profile location?

There are currently many messages on the server that are not being retrieved.

Try to delete popstate.dat. See http://kb.mozillazine.org/Popstate.dat

when I click "Get Messages" and get a "Add Security Exception" box and then click on "Confirm Security Exception"

Avast is intercepting your secure connection to the server. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

Tried exporting the Avast Mail Shield certificate and importing to Thunderbird's certificate store as suggested in the above post (https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST ) but still the same problem. ...Also note that in my original post, Thunderbird worked and retrieved messages when first installed WITH Avast already installed.

OK got it working. Here's the details in case it helps anyone or makes any sense to anyone: Disabled the Avast "Mail Shield" to stop Avast from intercepting the Thunderbird's secure connection to the server. After this I could click on "Get Messages" and for the first time would see at the bottom left of Thunderbird " " .....Connected to mail.comcast.net ...." But it would just hang there and not get any mail. I then went into the Thunderbird profile and deleted all files from the profile and then went to my backup profile and copied and pasted all of these back up files into the current Thunderbird profile location. After this, everything worked OK. What still puzzles me is: 1) When I first installed Thunderbird, Avast was already installed and Thunderbird worked great and would retrieve messages; it was only after I copied and pasted my backup profile into Thunderbird that I started getting the security certificate warning from Avast. 2) Not sure why deleting my Thunderbird profile files and then pasting the back up profile files into it (as mentioned in this post) helped because that was was I did at the beginning of this whole process (as mentioned in my original post).

Now will start a new post on trying to get Avast to work with Thunder bird. I did try the above suggestion, "Avast is intercepting your secure connection to the server. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_ Performance_Issues#AVAST" but had no success --There is a risk of running Avast with the "Mail Shield" permanently set to off, right? This isn't something I should do permanently?

And reading over my above post another item that's puzzling is even with Avast uninstalled, I had problems getting with the security warning which makes no sense to me.

There is a risk of running Avast with the "Mail Shield" permanently set to off, right?

I have no idea about Avast, so I'm not sure what exactly "Mail Shield" does. Assuming it means scanning incoming and outgoing messages, then that's pretty much useless. Since that seems to include intercepting your encrypted connection to the server, it also means Avast can see your email password in the clear, so it is breaking the secure connection between your computer and the server. As you found out the hard way, it causes more trouble than it helps. In any case, you're still protected by the anti-virus real-time scanner.

This isn't something I should do permanently?

Leaving Avast "Mail Shield" permanently set to off is what I'd recommend.

Now will start a new post on trying to get Avast to work with Thunder bird.

I think you're wasting your time. This forum is full of complaints about Avast.

Yep, you're probably right ....and as an addendum, I uninstalled Avast and reinstalled it and now do not get the security warning even though the "mail shield" in on. A lot of this sure doesn't make much sense to me. What antivirus/malware protection do you like? I had Norton as it was free with comcast but it seemed so resource intensive that I tried something else --Avast-- that seemed to have a lot of descent recommendations.

I don't think there's a point giving a specific recommendation for any anti-virus software. Use what works best for you.