Let's monitor the size of data we're adding to content-settings, for phishguard pings, and find a better place if they turn out to be large.  

brettw's suggestion on the doc:
"I'd say If this is more than a couple of KB of data total, we should probably find a better place than the prefs for it to live on disk. The prefs are a pretty inefficient way to store data and makes everything slower. It doesn't seem like you need the "synchronously available at startup" properties of prefs. The main place to put random stuff like this is the WebDatabase (components/webdata) which is where things like autofill data are stored."