How to Prevent Postbox From Expanding Mailing Lists In the Compose Window

By default, Postbox expands mailing lists in the addressing widget of the compose window.

To disable this feature, you need to set a hidden pref. Open the Postbox Config Editor and type pb.msgcompose.explode_mailing_lists into the "Filter:" textbox and double click the row for  pb.msgcompose.explode_mailing_lists to change the value from true to false