Thanks for the write-up, your diagnosis was correct, and our apologies both for the breakage and for the delayed reply to your support request (we've answered it by email).

The explanation you asked for: Caramel recently went from supporting a handful of stores to all stores, which requires the "access your data for all websites" permission. Firefox does not enable that permission automatically, and with it off the extension can't reach our coupon service, the "couldn't load coupons" message you saw.

To fix it: Add-ons and themes → Caramel → Permissions → enable "Access your data for all websites."

Nothing new is collected, the permission lets Caramel read the store page you're on to find the coupon box, which is how it has always worked. Mozilla now requires this to be declared explicitly, which is the notice you saw. An update is on the way that detects the missing permission and offers a one-click enable.