The config values you want to be looking at are the following:
// --------------------------------------
// # RFP
// --------------------------------------
defaultPref("privacy.resistFingerprinting", true);
defaultPref("privacy.resistFingerprinting.block_mozAddonManager", true);
lockPref("browser.startup.blankWindow", false); // breaks RFP windows resizing
Try commenting out / setting to false & restarting the browser. If that solves the issue you can put this into the override file to make it stick (if thats what you want) 