It used to be possible to find it in either Tools > Options > Advanced > General > Config Editor or Preferences > Advanced > General > Config Editor.
How can I open it now?
For me there is a "Config Editor..." button in the Preferences window if I select "Advanced" (on TB 52.1.1).
The "Advanced" tab has four sub tabs:
In the screenshot above you have the "Certificates" sub tab open. Click "General", and you'll hopefully see the "Config Editor..." button.
Turn off your Thunderbird and open
and delete it (or set it to
0 which is the default value).
Generally, look for
prefs.js and find out which line is the culprit.
Thanks to mook765.