See also bug 508250.
The Forward Extension adds Inline / As Attachment option to the Thunderbird Forward Button and Message context menu. It allows you to more easily choose how to forward your message, and switch between forwarding inline and as attachment.
I did not invent the feature to Forward Inline. This is a standard feature of Thunderbird, which can be configured through the field "Forward Messages" under "Tools → Options → Composition → General".
This feature has always been available in the main menu "Message → Forward As". My extension just adds this feature to the Forward button and Message context menus, making it more easily accessible.