It's probably best to just disable sessionstore if you don't want it, as overwriting fsync would disable it for all operations Firefox might use, not just sessionstore.
That's exactly what I wanted to achieve; I didn't have the time or inclination to work out exactly which part of Firefox is responsible.
In my case it was not SSD wear I wanted to reduce, but the short pauses during interactive use.
Take a look at libeatmydata. I'm doing something practically identical