Start *Windows'* safe mode with networking enabled - win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still In Windows safe mode, start thunderbird in safe mode - https://support.mozilla.org/kb/safe-mode-thunderbird

Does problem go away? Please reply to let us know results.

still happens.....sometimes the "not responding" is only about 30 seconds, and sometimes it is a few minutes

please see https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

Wayne Mery said

please see https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

Please post your results

I don't have time for this mess. when i 1st started using Thunderbird, it worked fine.....now a couple of years later and several updates later, it is the most unreliable mail program i have ever used. it crashes on startup sometimes; it decides not to respond quite often and it is a pain in the neck I have too many things to do to try and troubleshoot for Mozilla---YOU should be troubleshooting and fixing your own programs. I can go back to Live mail or just use browsers which I detest, for email

Approx 25 items are listed at that reference of which approx half are caused by non-Thunderbird sources. And the experience of support volunteers is most users issues are caused by non-Thunderbird sources.

If you choose to stay with Thunderbird, a volunteer can try to help you, but we cannot see what is happening on your system - only you can. But you are of course welcome to go elsewhere.

You did not mention a crash in your original post. If you post your crash ID here I can determine where the crash is occurring. See https://support.mozilla.org/en-US/kb/mozilla-crash-reporter#w_viewing-crash-reports