The problem was solved after a windows update.

remoz said

Thunderbird stopped responding from the very second of starting it if it is starting online. When started offline (by shift-clicking the icon, Choose user profile, and selecting the start offline option) it works normally, until I go online. The second it starts online (or goes online from offline state), it stops responding, I get a "Not Responding" in the title, the CPU usage goes up, and the only way to stop it is to kill the process. I am running the latest Thunderbird (52.8.0; it identifies as 32 bit, this is the only installer I found) on windows 10, 64 bits. I already did the obvious things -- started a new profile, uninstalled Thunderbird and re-installed it, uninstalled it and installed the previous version, tried to run Thunderbird in "compatibility mode" -- nothing helps.

Wow, that is really interesting.

If the problem returns or if you develop a theory of what specifically changed to improve performance please update the topic.

Several hours before this happened, I uninstalled a pre-installed trial copy of Microsoft Office. Perhaps the removal caused some problem, and then Windows Update fixed it. I am not knowledgeable enough to tell whether Thunderbird relies on windows components that other networked applications don't, and whether such components may have to do with MS Office, or with something that the MS Office uninstallation process can damage. It probably did have to do with some OS component, perhaps a driver, because it seems that Windows Update was what fixed the problem.

Wayne Mery said

Wow, that is really interesting. If the problem returns or if you develop a theory of what specifically changed to improve performance please update the topic.