But you should be careful when you try to modify layout.css.devPixelsPerPx If it's really but hugely exaggerated ... Like the configuration editor is no longer accessible In this case, the solution is to stop Thunderbird start Thunderbird stop Thunderbird Then in the user profile https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data modify prefs.js level layout.css.devPixelsPerPx and reset to default value -1.0 and save prefs.js and start Thunderbird
