cachestatus-firefox
This is the code repository for the Cache Status extension/addon for Firefox. Unfortunately, Firefox has removed the way this add-on accessed the cache, starting in version 57, and it appears there is no new way to work with the cache system from the add-on layer at this point.
I have posted a request on the Mozilla bug system, so if there's ever support in the future, then I will revisit this and see if I can get an updated add-on.