Never heard of that error before, so I tried google and one of the first things that popped up was one of my least favorite antivirus products. https://forum.kaspersky.com/topic/pr_connect_reset_error-on-firefox-35402/ Yes the issue is Firefox (Thunderbird and Firefox share basically the same code for the connection and network) Most of the fixes for Firefox also work with Thunderbird. Just the user interface to get to the settings is different. I did have a look here https://cheapsslweb.com/blog/how-to-fix-pr-connect-reset-error-in-firefox and a few other sites with articles mentioning the error and it looks like they have a fairly standard diagnostic process before pointing the finger at the antivirus. 1. Clear the cache. (There is a button for that at the top of the troubleshooting information on the help menu.) 2. Disable security measures – antivirus & firewall (generally here, unless there is some clear reason, we do not suggest disabling Windows defender) 3. Disable and virtual private network(VPN) and proxy 4. Disable addons.