[Thunderbird] Extension does not toggle when only the Write window is open

[cyrusdavid]

When I click on the Toggle button it just highlights the message:

Thunderbird version: 24.4.0
MD-H version: 2.10.0

It works though when I open the main thunderbird window.

[adam-p]

I think this is due to issue #141. It should be fixed in the next release. (But leaving this open for now pending more testing.)