Containers don't include the HSTS response cache (https://wiki.mozilla.org/Security/Contextual_Identity_Project/Containers#What_is_.28and_isn.27t.29_separated_between_Containers) and as such it's not removed when containers itself are removed. Unfortunately no way for Add-ons to clear that cache in any other way. The bug where this decision was made can be found here: https://bugzilla.mozilla.org/show_bug.cgi?id=1253006