You may sometimes need to view or edit Thunderbird preference settings which are not accessible through the normal user interface. You can use Thunderbird's "config editor" to do this.
To access the config editor, go to Thunderbird > PreferencesTools > OptionsEdit > Preferences, select the Advanced panel, select the General tab, and click Config Editor….
The first time you do this, you will get a warning telling you to be careful.
Uncheck the "Show this warning next time" box if you don't want to see the warning again, then click I'll be careful, I promise! to proceed.
In the config editor, you can type a string in the "Search" field to filter the list of preferences to only those that contain the specified string in their names or values.
Preferences that have been changed from their default values are shown in bold.
To edit a preference, double-click on it, or right-click on it and select Modify (for true/false preferences, select Toggle). To reset a preference to its default value, right-click on it and select Reset.
In rare cases you may need to add a new preference. You shouldn't need to do this unless you are instructed to do so by documentation for Thunderbird or one of its add-ons. To add a new preference, right-click anywhere within the list of preferences, select the New menu item, and then select the appropriate sub-menu item for the type of preference you need to create. To delete a previously added preference, Reset it as described above.
Modifications to preferences happen immediately, which means you don't have to click a Save button. However, some parts of Thunderbird or many add-ons only read their preferences on start-up, so it is good practice to restart Thunderbird after editing preferences for your changes to take effect. Note: A restart requires all windows to be closed, including the config editor window and OptionsPreferences windows in addition to the main Thunderbird window!
It is also important to know that when you restore a preference to its default value, Thunderbird does not store that preference in the preferences file. It is only non-default preferences that are stored.