This could be because of security measures by windows or any other security provider you use. In case you use Windows security, follow the instructions mentioned here to fix Thunderbird not receiving emails. Open Settings and click on Update & Security, click on windows security next and navigate Virus & Threat protection. You will find manage settings option right under it, click on it then on Add or remove exclusions, and finally click on Add an exclusion to select Thunderbird. Related Issues Thunderbird Email Not Responding How to Setup Thunderbird for Yahoo Mail

Most of the time, users face the issue of Thunderbird not responding because of outdated version causing problems. It is always a nice idea to use the latest version to avoid incompatibility concerns.