1) Download G-Hub Pro from the client center: http://www.schiffner.com/index.php/purchases/
2) Open Mozilla Thunderbird and click the menu Tools > Add-ons.
3) Drag and drop the G-Hub Pro installation file you downloaded in step #1 on the Mozilla Thunderbird add-on window. Alternately click the gear icon at the top of the add-on window and select “Install add-on from file”. Locate the file you downloaded in step #1.
4) An installation confirmation box will appear and the “Install Now” button will initially be greyed out. Once the button becomes available click it.
5) Restart Mozilla Thunderbird to complete the add-on installation.
6) Right click the Thunderbird toolbar and select “Customize…”
7) Scroll down in the dialog that opens and locate the button associated with the Google service you wish to use.
8) Drag the button to the location on the toolbar you would like it at.
9) Close the Customize dialog.
10) Click the button you added to the toolbar to launch its associate Google service.