Hi there,
This addon uses content-script API from webextensions API suite, which requires the aforesaid permissions. And this doesnt send or store any of your data, here is the code for this addon
https://github.com/tsl143/jsDemos/tree/master/measureIt_webext

I actually second your thought and even I find that permission intimidating, I have mailed Firefox team for same, hopefully we get a better solution.
Thanks again for input :)

UPDATE-
updated addon with different approach and without requiring website-data permissions :)