Rated 4 out of 5 stars

Must have addon, but need "Reply All" button

This review is for a previous version of the add-on (2.0.7). 

You can right click on the header toolbar and "customize" it: Then you can add or remove buttons. The reply all button is in the default set (click the corresponding button in the customization dialog or add it by drag and drop).

Rated 5 out of 5 stars

All it fine for very minimalistic mode. But lack of buttons for reply and forward is minor inconveninece that could be resolved as buttons in tabbed area on the right (close to calendar and address book)

This review is for a previous version of the add-on (2.0.1). 

You can either use the two line mode (use the context menu "CompactHeader preferences" or change the settings in the addon tab) to see the header toolbar. Or you can change the position of the toolbar either to left or right then it will be a "sidebar". The the requested buttons are always visible.