This is the first time that I want to write my own TB add-on. I want to create an extension that detects when a mail comes from a specific sender. These messages usually include a table with some information and I want to export them. For this purpose I want TB to show an additional button next to the forward, reply, archive etc. buttons. I found that this might be done via an overlay for the msgHdrViewOverlay.xul. I created an overlay and it showed up when I used the “customise” option of that toolbar. However, I want that button to be shown automatically when a mail from that sender has the focus and the button not to be shown at all when the mail comes from another address. Can you help with some advice how to do this?