Users of the desktop email client Thunderbird can either use the options provided in the options of the program or the about:config dialog window to change and tweak the email client. Mail Tweak is an extension for the mail client that provides a third option. The add-on is provided for Thunderbird 2 and as a preview for Thunderbird 3.
It provides access to more than 30 tweaks to change settings and options in Thunderbird. The Mail Tweak options are displayed in a scrollable list upon installation. Each configured entry is displayed with a checkmark next to its name while the currently inactive tweaks use blank checkboxes. A click in a checkbox activates or disables the selected tweak.

Tweaks that are provided by the extension include useful ones like changing the number of address lines that are displayed by default when writing emails, changing the default message font family, a collected reply option to reply to all selected messages in a single message or automatically hiding the folder pane in Thunderbird.
All tweaks including a short description of their functionality are available at the website of the extension developer. The website contains download links for both versions of Thunderbird and users should ensure that they download the correct version for their email client.
Mail Tweak offers several interesting settings for the email client that many Thunderbird users will find useful. The developer is constantly improving the extension to add new tweaks and optimize existing ones. Definitely an extension to look out for.
Update: The extension has not been updated for newer versions of the Thunderbird browser. The last supported version supports Thunderbird 3.0, and not newer versions of the program. It may still work though, but it seems as if the program is no longer updated by its author.
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it doesnt workon Thunderbird 8.0