Found this here: (Among a lot of other good stuff about Mint) https://sites.google.com/site/easylinuxtipsproject/3
9. By default, Firefox writes a lot to the hard disk. This costs system resources.
You can sharply reduce the write actions of Firefox by disabling the session restore feature, which remembers what pages were opened if Firefox experiences an unexpected shutdown (read: crashes). This feature is neat, but causes many disk writes. You can practically disable it as follows:
Type about:config in the url bar of Firefox and press Enter. Click the button to accept the risk.
In the search bar, type: sessionstore
Double-click on the item called browser.sessionstore.interval. The default interval is 15000, which means 15 seconds. Add three zeroes to the existing value, so that it becomes: 15000000 and click the OK button (note: adding too much zeroes causes an error!).
Close Firefox and launch it again.