what does the actual certificate say? Who is shown as the issuing authority?

I have a similar problem. When I click "Get Messages" in tbird I'm rejected: the certificate on the Xfinity server isn't valid. I override the error message (there's a checkbox on the error dialog box to allow me to override) and then I right-click on my email address in the far-left panel of tbird (the address right under "New Message"). On the resulting menu I click "Get Messages" - the top item. Then my messages come in. I get my messages that way through an entire session and I don't see the error again that day. The next day I have to repeat my override and right-click process; and yes, it's an annoyance.

But today I came across Norton article that suggests it may be a Norton 360 problem. Go to this address to see this article and their suggested solution:

"Troubleshooting email client warnings about invalid server certificates": https://support.norton.com/sp/en/us/home/current/solutions/v20240108183457190?q=email+certificates&pg=1&tab=kbarticle&site=nrtn_en_US&client=norton

I've tried the fix but I don't know yet if it works. It seems the error occurs only once per day on my computer and I can't make it happen a second time. So I won't know until tomorrow if this fix works. By the way I got to this article by clicking on my Norton 360 desktop icon and then searching for "email". After wending my way through the anti-virus forest I found the article. Good luck!

Frank

Here I am again (I posted the reference to the Norton 360 article yesterday). Today is the second day and I'm not getting the certificate error any more. So Norton's fix appears to have worked. Tbird is now restored to its original operation: when I start it up the program automatically downloads my new messages, and during the day I can click on the Get Messages button and all my new messages are downloaded. I don't see the "bad certificate" error message any more.

Frank