To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

I am having this problem as well in the same operating system. I just tried your suggestion of running Thunderbird in safe mode and it is now even worse. Not is it slow in getting and sending emails but it is currently hung up on moving emails from the inbox to a Storage folder. I am trying to reduce the size of my inbox as that was a suggestion on another site.

I have tested my email address (Hotmail) on their site (Outlook) and it works fine. Also tested on my smart phone and Ipad and Ipod and all work fine.

My AV program has been running on this environment since I purchased this computer with W10 installed. I am having no problems accessing the web or other computers on my home network. No new software has been installed since before this problem started. As far as I am concerned there is a problem with Thunderbird and since they are "going through some system changes" makes it all the more likely. I would say that the ball is now in Mozilla's court.

KuchleiT said

I have tested my email address (Hotmail) on their site (Outlook) and it works fine. Also tested on my smart phone and Ipad and Ipod and all work fine.

Slow performance with a Hotmail account?

https://support.mozilla.org/en-US/questions/1156972#answer-961439

email is NOT loading in a timely fashion.....takes 45 sec for an email from spectrum to load so I could read it back to spectrum....this last update is problem-atic....please fix

Chicopee said

email is NOT loading in a timely fashion.....takes 45 sec for an email from spectrum to load so I could read it back to spectrum....this last update is problem-atic....please fix

After a long and sometimes frustrating email war with Microsoft support I finally got them to admit that they were having server issues. They provided a fix which I have been using now for over a month and everything is back to the good normal. Below is their solution; __________________________________________________________________________ I have received some information back from our agents after they went through your issue. They would like you to use this information with the account to test if we see a change in how that is acting for you. Due to outlook.com server migration, the mail server settings are changed to Server name Port Requires SSL (TLS) Incoming Mail (IMAP) Server: outlook.office365.com 993 or 143 Yes (always encrypted) or Yes (STARTTLS) Incoming Mail (POP) Server: outlook.office365.com 995 Yes (always encrypted) Outgoing Mail (SMTP) Server: smtp.office365.com 587 Yes (STARTTLS)

Get back to me if you see any issues with that still after so we can have this looked into further.

Regards Outlook.com Support