executeScript() for thunderbird/lightning/tasks? (in tbird/addons)

Hello,

I’m trying to write an add-on to thunderbird/lightning/tasks to delete a
UI element that gets in my way:

/html/body/vbox/hbox[1]/tabmail/tabbox/tabpanels/vbox[2]/hbox/hbox/vbox[2]/hbox/box[1]

but I can’t get executeScript() to be injected in the tab. I can find
the “Tasks” tab, but it has no URL, unlike other tabs. Is it forbidden
to inject a script into that tab?