The "restore interval" write function of Firefox (and also Chrome) produces a lot of writings over the day, according to this page.
Go to about:config and search for the value browser.sessionstore.interval
This is by default set to 15 seconds and causes the hige integrated written bits over a day - more 10th Gibibytes (GiB) per day. The time is given in millseconds.
I've set it to 30 minutes, so in my configuration
browser.sessionstore.interval = 1800000
Restart the browser and that's it 🙂