-- Win10 Pro 2004 (happened in 1909 also) 64bit
 -- Using Win10 roaming profiles (but saving actual mail folders on C drive - problem also happens when storing in default location in roaming folder)
 -- Thunderbird 78.2.1 (and previous) - no addons

 Restart in safe mode, addon disabled
 Delete files:
     parent.lock
     session.json
     xulstore.json 
 Delete everything in:
    C:\Users\<WinUsername>\AppData\Local\Thunderbird\Profiles 

Environment: -- Win10 Pro 2004 (happened in 1909 also) 64bit -- Using Win10 roaming profiles (but saving actual mail folders on C drive - problem also happens when storing in default location in roaming folder) -- Thunderbird 78.2.1 (and previous) - no addons Very frequently some users launch Thunderbird and only a blank window appears. The panels exist but there are no folders and account/profile is broken. The options hamburger menu button is there but clicking on it does not generate the menu. Cannot access the troubleshooter window from the help menu either - ie. click Help -> Troubleshooting Information does nothing. Troubleshooting steps tried, all result with same blank window: Restart in safe mode, addon disabled Delete files: parent.lock session.json xulstore.json Delete everything in: C:\Users\<WinUsername>\AppData\Local\Thunderbird\Profiles Only steps that work is to create a new Thunderbird profile and set up all mail accounts again.