A friend has a copy of Thunderbird Portable installed on his computer for checking emails. After a month's using, it's getting unstable and suddenly one day it stops loading. He called me for help and here're some findings, then a possible FIX for you..., besides an overview of my troubleshooting steps.
The first thing I check, Task Manager, I see one instance of "thunderbirdportable.exe", but two instances of "thunderbird.exe". Obviously, these are from previous freeze, and he double clicked on the Thunderbird icon again. Stopping all these tasks won't help.
Then, I used PROCMON.exe (from Microsoft) to monitor what the thunderbird.exe is actually doing, and it seems that it's doing nothing...or maybe waiting, or even it's hanged...
I copied the whole Thunderbird folder (because it's a portable version) to my PC and started it. It started perfectly fine! Well, this is something related to the "SYSTEM" of his computer, not just with the Thunderbird...
I started to suspect the way Thunderbird is working, TEMP FILES!!! I checked and found out that there're 4,000+ temporary files in the temporary folder "C:\Users\Username\AppData\Local\Temp", which is also the folder that Thunderbird is storing some temporary files*. Most of the temporary files in this folder reported 0KB in file size.
SOLUTION: Delete temporary files from the folder "C:\Users\Username\AppData\Local\Temp". Then, restart Thunderbird again. Or, reboot and start Thunderbird. (if you do not know how to End Task of all Thunderbird processes)
*Notes: When Thunderbird starts, a temporary folder is being created in the user's TEMP folder, you may identify this folder by looking for a file with name "splash.jpg" inside the folder.