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. 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. Modifications to preferences happen immediately. To delete a previously added preference, click the trash icon following the entry

Click on the Add to add the preference.

Select the appropriate preference type (number, string or boolean) from the offered options.

Type or paste the preference name into the search box.

You shouldn't normally need to do this unless you are instructed to do so by documentation for Thunderbird or one of its add-ons.

In rare cases, you may need to add a new preference. Once the edit is complete, save your changes by clicking the checkmark For text preferences, double-clicking on the preference will enable in-place editing.

To edit a boolean preference ( true/ false value), click on the Toggle button. There is a revert arrow on the end of the entry to revert the setting to the default value Note: Preferences that have been changed from their default values are shown in bold.

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