W7x64 FF45.0.1
The path you enter for the read mail button option cannot have quotes:
This does not work: "C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe"
This does work: C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe

The select button does not work to obtain the path. A path copied from a working shortcut will likely have quotes, and a reminder to remove them (or bug fix) would help.