What steps will reproduce the problem?

1. Visit https://www.shinesty.com
2. Should be your first visit, should get a pop-up for emails/newsletters
3. Provide any random email address, "SomethingSuperSpammy@gmail.com"
4. Click on Submit
5. Visit the Account login page by clicking the 'person' icon in the upper right.
6. Verify that the email address "SomethingSuperSpammy@gmail.com" appears in the email address field of the login form.

7. Open an Incognito instance and visit the same login page.
8. Verify that the email address "SomethingSuperSpammy@gmail.com" appears in the email address field of the login form.

9. Open Firefox and visit the same login page.
10. Verify that the email address "SomethingSuperSpammy@gmail.com" appears in the email address field of the login form.

11. Open an instance of Private Browsing in Firefox
12. Verify that the email address "SomethingSuperSpammy@gmail.com" appears in the email address field of the login form.


What is the expected result?
Firefox, Incognito, and Private Browsing (FF) should not be auto-populating that email address.


What happens instead of that?
The email address is being pre-populated by the site which means I'm being tracked across platforms. This shouldn't be possible. 

Please provide any additional information below. Attach a screenshot if
possible.

I've reproduced this on my home computer, my phone, and my pixel book. Tracking is limited to the device.  Clearing cookies/cache/etc...hasn't helped.  I've not been able to repro it multiple times on the same device, it seems to start with that first 'email sign up' box.  

UserAgentString: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.77 Safari/537.36