Error pages are broken in TBB ff52 nightly with security slider being on "Medium" or "High"
Go to a non-existent website http://nonexisssstentwebsite.com/
Or go to a website that doesn't support HTTPS with HTTPS: https://mysite.com/ (the "Go Back" button works, and the "Advanced" one doesn't)
Trac:
Username: blockflare
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information