I’m really disappointed by the new Avast version 8. I was quite satisfied user of Avast Free until v. 7, but after the update to Avast 8, SSL mail accounts in Thunderbird stopped working, until I enabled import of Avast certificate (which I suppose was a big mistake, as below, but without that the SSL accounts were not working - I spent really lot of time figuring that, it was quite annoying). After some additional issues with Avast 8 (at the end impossible to finish the free registration, so I couldn’t renew my registration), I finally decided to uninstall Avast and replace by Microsoft Security Essentials.
But after I uninstalled Avast 8, all my SSL mail accounts in Thunderbird stopped working, Avast must have blocked them somehow! Is there a way to recover the ability to connect to SSL mail accounts in Thunderbird? I tried many things found in this forum, also removed the imported Avast certificates from Thunderbird. Tried even the special Avast uninstaller application and fiddled with it in the Safe Mode, still didn’t help!
I really do not want to reinstall my Windows. I’m quite pissed off, I know that it is a free version, but even then blocking the users operating system and/or mail client is not a good work. This is really lame work of Avast programmers. 
(by the way, after uninstall of Avast somewhat around 30 important Windows Vista security updates were showing, some were pretty old - that was quite surprising, as I have automatic updates switched on, it seems that Avast also blocked those Windows updates somehow)
With the older versions of avast you would of had to disable the SSL security inside of Thunderbird for avast to scan your mail, this is not necessary with the new avast 8 versions as it will scan SSL, so the SSL security adjustments you made originally inside of Thunderbird for avast would now need to be set back to there normal settings and your mail will function again.
In fact I didn’t disable SSL in mail accounts in Thunderbird with the older version of Avast - there was a warning message of Avast about that (don’t remember the exact phrasing) but I selected to ignore this and not check my SSL mail account in Avast. But in Avast 8 it was no more possible, as when I disabled SSL mail account checking in Avast (to not have to override the mail provider certificates by the avast ones) the e-mails were not downloaded at all, so I had to enable SSL mail checking back in Avast and override the mail provider certificates by the Avast ones (I suppose Avast uses the “man in the middle” technique to be able to check SSL e-mails). But after uninstalling Avast the situation seems to be the same like when I disabled the SSL mail checking in Avast - Thunderbird seems to try to connect to the mail accounts (shows e.g. “Connecting to pop.gmail.com”), but then the connection ends without downloading any e-mail or any error message.
The funny thing is that even when I remove all stored passwords in Thunderbird, it doesn’t ask for the password. Just drops the connection without any error message.
What firewall are you using?
Also please run RejZoR’s avast removal tool from here https://skydrive.live.com/?cid=94a12102e5094675&id=94A12102E5094675!952&authkey=!AHu_KR3cJN1jHt4 to delete any remnants