Thunderbird support?

We could use InvertImage in Thunderbird too!  I was excited when the add-on installed without issue, but it has no effect and doesn't function, don't see an "Invert Image" option in any context menus... Do you know how code could be modified to work in Thunderbird?  I could work on a PR if i get some initial guidance, seems like it could be an easy fix though.  Thanks