The website is using IndexedDB, for which MDN writes:

IndexedDB is a way for you to persistently store data inside a user's browser. Because it lets you create web applications with rich query abilities regardless of network availability, your applications can work both online and offline.

Not clearing it sounds like a bug in Firefox indeed, but apparently the developers feel otherwise. Like in March 2015, someone wrote:

But even when you delete all your history information the data from IndexedDB persist. The right way to delete this data is by going to about:permissions address, look for the domain and pressing the Forget About This Site button.

While about:permissions does not work in my Firefox 55, going into Tools, Page Info, Permissions I get the button "Clear Storage":

Even worse, neither the greyed-out "Use Default: Always Ask" in the above screen capture, nor enabling "Tell you when a website asks to store data for offline use" in settings, Advanced, Network, have any effect to avoid storage:

It seems the following from August 2011 may still apply (where "[only]" is added by me):

By default in Firefox 4, a site can use up to 50MB of IndexedDB storage. [Only] If it tries to use more than 50MB, Firefox will ask the user for permission [...] In Firefox for mobile devices (Google Android and Nokia Maemo), Firefox will [only] ask for permission if a site tries to use more than 5MB [...]

To disable it altogether, go to about:config and disable dom.indexedDB.enabled . However, beware that such might affect plugins/add-ons as well, which seems to be why some want to remove that option, for which someone noted in May 2016:

Until IndexedDB is handled in the same way as cookies with respect to accepting/clearing and third-party behavior, this pref should exist.