I too had major problems with Thunderbird yesterday:
My problem was on one Wintel PC (2 other PCs worked perfectly) - I could send, but not receive, emails
- 2 degrees gigabit fibre (no outages)
- Windows 10 (with latest updates)
- ESET Smart Security (with latest updates)
- Thunderbird (with latest updates)
- @gmail.com email address
I was unaware of this fresh topic, and fixed it the hard way:
- I completely uninstalled Thunderbird (using Revo Uninstaller)
- Fresh install with Thunderbird v.78.5.0, using all original settings.
It now works perfectly. 🙂