To avoid e10s performance problems, Mozilla has removed restore tabs on demand checkbox from Firefox 48. With Firefox 13 release, Mozilla has set their browser to restore tabs on demand by default, as a result of that, you can notice ‘Don’t load tabs until selected‘ preference checked in ‘Tabs’ options. With this, Firefox only loads the tab you’re facing when it starts or restores the previous session, the other tabs will be loaded as you click them. This improves the Firefox startup performance.
With e10s aka multi-process Firefox, the company has noticed the feature as broken in Firefox 48 when the restore tabs on demand was disabled. To avoid this, the option has been removed and the checkbox was kept hidden. At the moment, browser.sessionstore.restore_on_demand preference has been reset to true.