From MozillaZine Knowledge Base
Seem like DOM inspector cannot be used for changing the DOM in Thunderbird / Firefox 45.0 anymore. This is a vital functgion for us XUL addons and theme developers.
I suggested a patch, but it wasn't picked up yet: https://bugzilla.mozilla.org/show_bug.cgi?id=1267157
unfortunately I was not able to build a functioning version myself can somebody add a section on how to patch DOMi for Fx/Tb 45.0 and later?