Enormous green disk

When I open a Google Docs document anonymously from my desktop computer (Windows 7 + Firefox esr 52), I always get this enormous green disk at the bottom right:

Big green disk Clicking on it has no effect, except for the smaller white “Explore” button inside. Clicking on that white “Explorer” button opens the pivot table tool.

How can I get rid of that disk so that it won’t reappear in the future when I browse Google Docs anonymously?

I guess it has something to do with the “Explorer” feature, but whoever thought it would be a good idea to make it take a sixth of my total screen space must have been high at the time.

[Edits]:

By “anonymously”, I mean “not logged into my Google account”, not “using the Private Browsing feature of Firefox”.

RubĂ©n’s answer implies that it could be done by upgrading Firefox. So I should add that I don’t want to upgrade Firefox. Maybe something about cookies keeping my preferences while browsing Google Docs without being logged in Google?