The Window property devicePixelRatio returns the ratio of the resolution in physical pixels to the resolution in CSS pixels for the current display device. This value could also be interpreted as the ratio of pixel sizes: the size of one CSS pixel to the size of one physical pixel. In simpler terms, this tells the browser how many of the screen's actual pixels should be used to draw a single CSS pixel. 
When the pref "privacy.resistFingerprinting" is set as true, we spoof the value of window.devicePixelRatio as 1, regardless of the real ratio. This behavior increases the protection of user privacy by reducing the entropy of the browser client fingerprint. However, it causes a side-effect that some web pages become blurred with the spoofed value, such as Google Spreadsheet. 
Changing the spoof value as 1.5 or 2 might resolve the blur issue, but we need to make sure such change won't cause other side-effects or breakage. This bug is filed to track the investigation work.