Delete xulstore.json from the needed profile, then restart TB in normal mode. Result? If that doesn't work, you may have to disable hardware acceleration manually in prefs.js, if it was enabled in normal mode. Open prefs.js in a text editor, edit the line user_pref("gfx.direct2d.disabled", true); if it is currently false, or add that line if it's absent.

http://kb.mozillazine.org/Safe_mode

There is no xulstore.json file in the relevant profile folder. Edited prefs.js, by adding user_pref("gfx.direct2d.disabled", true);

No change. However, starting the profile manager, selecting the needed profile and starting the application from the profile manager resulted in an error (could not be replicated afterwards).

Windows log:

Faulting application name: thunderbird.exe, version: 78.6.0.7650, time stamp: 0x5fd3965a Faulting module name: nvd3d9wrapx.dll, version: 23.21.13.9125, time stamp: 0x5aab6c27 Exception code: 0xc0000005 Fault offset: 0x00000000000028bb Faulting process id: 0x2348 Faulting application start time: 0x01d6d3efc360c457 Faulting application path: C:\Program Files\Mozilla Thunderbird\thunderbird.exe Faulting module path: C:\Program Files\NVIDIA Corporation\CoProcManager\nvd3d9wrapx.dll Report Id: 0626878c-3fe3-11eb-96b3-3c970e22b80f

If you don't see xulstore.json in the profile folder, you're looking in the wrong folder, as this file exists in every TB setup. The profile folder is in this location:

C:\Users\username\AppData\Roaming\Thunderbird\Profiles\randomstring.profilename

The error suggests you might benefit from updating or downgrading the graphics driver.

Sorry, but I don't have a profile in the default location. My only profile is on another partition.

Anyway, I could finally start TB after disabling "Use hardware acceleration when available" (while in Safe Mode) in Options/General.

Thanks for your help!