Can you post a screenshot of the error? http://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

When this box is up, I can not close TB or access the menu bar.

Please inspect the certificate (see the screenshot attached for instructions).

What's the issuer of the cert? Please also post a screenshot of the certificate viewer window.

Is this what you are looking for?

When connecting to pop.shaw.ca the server offers a cert issued by dbm001.dcs.int.inet. This is a self-signed cert Thunderbird doesn't know about.

Certificate chain

0 s:/C=US/ST=N/A/O=Zimbra Collaboration Suite/OU=Zimbra Collaboration Suite/CN=dbm001.dcs.int.inet
  i:/C=US/ST=N/A/L=N/A/O=Zimbra Collaboration Suite/OU=Zimbra Collaboration Suite/CN=dbm001.dcs.int.inet

You should be able to create a security exception, so Thunderbird does remember the self-signed cert.

If that doesn't work, contact Shaw and ask them how to get the cert for dbm001.dcs.int.inet. You'd then need to import that cert into the Thunderbird certificate store.

OK????? How do I create a security exception? Do I have to put that certificate chain that is in your reply somewhere? I am not really familiar with all this or the steps involved.

How do I create a security exception?

By clicking 'Confirm Security Exception' in the exception dialog (your 1st screenshot).

Under Server/Location is a line highlighted in blue that says pop.shaw.ca:110. When I hit confirm security exception, the box blinks and remains the same.

I have no idea why creating the exception doesn't work for you.

Talk to Shaw and get the cert for dbm001.dcs.int.inet.

OK Thanks for the help. I tried all you suggested and it is still coming up with the add security exception. I am getting it on my mac's Mail and the TB installed there too. On my PC I thought it went away but after a restart but it came back too. So I guess its back to Shaw. Would deleting and reinstalling TB fix the problem faster? Anyway thanks again

Would deleting and reinstalling TB fix the problem faster?

No, it would be a waste of time.

Had the same problems, changed the connection security from STARTTLS to SSL (Account Settings, Server Settings, Connection Security), port was then changed automatically to 995 and problem has gone away.

Good luck!

I got this fixed by speaking to a Shaw techy who walked me through reinstallation of the Shaw Secure version of McAfee - for a different problem. My other problem (very clunky operation of TBird) was fixed by disabling real time scanning (techy suggested just upping frequency of scheduled scans instead), but the security exception thing stopped appearing after the reinstall of McAfee, apparently because McAfee now recognizes TBird as a preexisting program and not some sort of interloper that needs to be ultra-screened. Good luck. MaxineKL

Doesn't work for me, either, although so far it doesn't block any actions, only delays them (sometimes one click turns it off, other times it requires several attempts), and it only seems to affect my accessing the shaw webmail site from Thunderbird, not other TB operations.

In any case, shaw didn't have any answers for this when I asked them, but maybe the additional info about the cert will help. Shaw has only recently added TBird to the programs they officially support, so their techs have some catching up to do.

Okay, I got mine fixed. What this Shaw techy (and not the ones I contacted earlier) did was go into account settings (in TB tools) and in server settings, change the security to ssl/tls. This will change the port to 995. That should fix the problem (as it did for sskovrackette above - only I didn't understand their explanation until the Shaw techy did it)!

Have you tried this?

I have this same problem with Gmail, though only intermittently.