Click this button to turn off FPI when needed, and if you want to permanently disable it, you can remove the add-on.
Method 2: Set about: config
Access the settings page by typing about: config on the address bar and clicking Enter, then find firstparty, there will be two options appear.
Set up on about: config
To enable FPI, select true for privacy.firstparty.isolate by double-clicking on it. If you have problems accessing the website, you can choose false for privacy.firstparty.isolate.restrict_opener_access.
See also: Firefox 57 Quantum here, great Mozilla!