Hi try to go to mail shield → SSL accounts and add info there.
Also try to disable SSL connection in Thunderbird (as avast manages SSL connection if defined above).

I really don’t think that web shield has any thing to do with mail receiving or sending

As suggested above you must upgrade to avast 6.0.1091…
Several bug fixed and many new features…
Use the below links

Direct download links:

Free AV: http://files.avast.com/iavs5x/setup_av_free.exe
Pro AV: http://files.avast.com/iavs5x/setup_av_pro.exe
IS: http://files.avast.com/iavs5x/setup_ais.exe

Please download and do a clean install by using avast un-installer…

Please report back if problem solved or persist

Thanks & Regards
Ash