Hi, I have a toolbarbutton in thunderbird that shows a status of the
extension (gray image when extension is inactive and colored when it
is active). I can set the image of the toolbarbutton when it is on the
toolbar.

Now, how can I set the toolbarbutton image in the moment when
toolbarbutton is being added to the toolbar by the customize options?
How can I register to the event that the toolbarbutton is being added
to the toolbar?