4500: RFP canvas

1376865 was back ported to 59, so canvas prompt fatigue will be reduced. Note: the default for non-prompts is the same as if you clicked "Don't Allow" - i.e it serves up a 10x10px white square

user.js

@@ -1570,8 +1570,7 @@ user_pref("privacy.firstparty.isolate.restrict_opener_access", true);
  ** 1382533 - enable fingerprinting resistance for Presentation API (FF57+)
       This blocks exposure of local IP Addresses via mDNS (Multicast DNS)
  **  967895 - enable site permission prompt before allowing canvas data extraction (FF58+)
-      FF59: Added to the site permissions panel (1413780)
-      FF60: Only prompt for canvas data extraction when triggered by user input (1376865)
+      FF59: Added to site permissions panel (1413780) Only prompt when triggered by user input (1376865)
  ** 1372073 - spoof/block fingerprinting in MediaDevices API (FF59+)
  ** 1039069 - warn when language prefs are set to non en-US (see 0207, 0208) (FF59+)
  ** 1222285 - spoof keyboard events and suppress keyboard modifier events (FF59+)