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.
How to enable Tor features in Firefox increases security when browsing the web Picture 2
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!