You may go to "about:cache" and look onto contents, there you can see URLs that are cached. And you can delete cache in "about:preferences" tab.

You may also inspect cookies and local/session storage data in inspector, open it (Ctrl+Shift+i) and look at "storage" tab. You can also delete there, but it isn't for browser cache, but for site-specific data.

I've tested kohl in empty FF profile - nothing suspicious. Few cookies that consume no space, few mb cache from start. Most of cache contents are images, although some scripts exist too. 

> There is no way I can get rid of it.

Clearing cache button must work. Automatic cache expiration works when expiry time is reached, it is mostly +1 month on kohl (images are managed through cloudflare anyway). You may go into your cache directory and delete them by hand, although file names are hashed keys. Cache located in ~/.cache/mozilla/firefox/PROFILE/... on linuxes and somewhere in AppData on windows.