Generally Thunderbird gets mail asynchronously (all accounts at once) that may be enough to trip Norton. A restart without the product enabled (safe mode with networking) would be needed for a conclusive answer.

Another possibility is you might be able to change the connections in server Settings > advanced for the imap account to a lower number so the connections all get a chance. The default is five, it may be your provider only allows that many so the next to connect misses out.

I tried both recommendations (not simultaneously) and the result is the same.

I double checked network settings and everything seems to be correct. As far as I can remember there was on change in my network settings. PS. I noticed that I have a delay and in Firefox. When I click a quick link , it delays but if I click on the address bar and press Enter it loads the page almost immediately.

Could that response caused by a windows update?

After the issue with firefox browser, I tried to figure out if there was any change to the installed software at my desktop. There was an update to a software that uses virtual network adapter. When I disabled that adapter then all the problems with delays at imap mail server and browser link delays disappeared.

So what I think was the problem. Multiple gateways problem. If you have an issue like me, check the network connections you use. One gateway is enough, mulptiple gateways it might cause a problem.

Thanks for your interest reading this post