• Start Windows in safe mode with networking enabled - https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode
• Still In Windows safe mode, start Thunderbird in Troubleshoot Mode - https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

Does problem change? (many users have found antivirus software causing problems)

Unfortunately my network devices are not supported in safe mode, so I could not try that.

However after rebooting back into normal mode, most of my accounts suddenly worked, with only yahoo accounts still not connecting. I then closed thunderbird, and when relaunching, none of my accounts work again. Back to square one.

I did try launching it in troubleshooting mode and disabling my antivirus, but still the same result.

What is your antivirus? What exactly did you disable?

I have Norton. Disabled both the Auto-protect and the Smart Firewall by right clicking the icon in the system tray.

So I deleted one of my accounts and attempted to re-add it. I type in my username and password, it says it found the configuration, but then when I click Done, it says that it cannot log onto the server, that either the username or password are incorrect, or there is a configuration issue. Even double checking the settings manually with what is listed on the help page results in the same error.

Out of curiosity, I opened up the new version of outlook and tried the same account there. After entering my credentials, it opens up a browser for me to log in and connect to the email client. IIRC, this is what Thunderbird is also supposed to do. It appears that isn't happening.

Any ideas on this? Is this a bug? Is there a workaround? How do I get this fixed? I really don't want to have to find a new email client to use.

Likely Norton.

It wasn't a problem until a few days ago. Been using Thunderbird with Norton for years without issue. That also doesn't tell me how I fix it.

Figured it out. Thanks.

Doesn't matter if our experience with Norton has been flawless for year, things interact and change and then don't work. We see these things in support.

Unfortunately I don't have direct experience with modern Norton to assist you.

Figured it out. Thanks.

Excellent!

Can you provide advice for the next person?

Here's the setting I turned off. Still don't know why it suddenly stopped working and required me to do this.

Security->Advanced Security->Antivirus->Email Protection.