cmichela, I use Thunderbird as a client in our small office (6 people) and ran into the same issues. After the automatic 'update' to v78.4.0 two of our users immediately experienced the same issues you're outlining. We confirmed settings, cleared cookies and passwords and even went so far as to wipe and reinstall the program from those computers and use backup profiles to restore. All attempts gave us the same result. In a last ditch effort I went nuclear on the program and removed all traces (except for the backup profile). Restarted the computer, installed TB again and manually entered the settings for the user account. Thunderbird immediately downloaded the waiting emails. The final act was to swap the profile from the newly-made fresh install with the backup profile. Once I restarted I was hopeful that it would operate properly but I was wrong. Once again it failed to download new messages. Now I'm thinking about making a fresh account and looking for a way to import my emails, archives and addresses from the backup. Maybe that will allow me to bypass whatever lingering settings that cause the original problems.