I'm not seeing these numbers either and I'm running Firefox 49. In the resource monitor, I see firefox writing to disk 10kB every 15 sec. I bet there is something else to the story of this guy.

Nightly Portable might have this particular setting changed, because the lifetime of random USB sticks is surely going to be much less than a real SSD.

i thought that too but the interval setting is at 15000 in the config, so maybe something else is optimizing it out?