Rated 5 out of 5 stars
This addon works great with both Firefox 45 and Thunderbird 45. I had to modify install.rdf to get it to work with Thunderbird though as it doesn't have the application ID.
If anyone is looking this is a great substitute for "Minimize on Start and Close"
If you find that after installing this addon that the "Delete" key has stopped working go into settings and uncheck the "enable:Delete" option.