• 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.