If you choose Restore the last session, the URLs that were open last time Google Chrome was.
Click I'll be careful, I promise! to continue to the about:config page. The about:config 'This might void your warranty!' warning page may appear. In the address bar, type about:config and press Enter. You can change it back through your about:config preferences (advanced users only): If you've allowed an earlier version of Firefox to simultaneously load multiple tabs in the background (offered as the 'Don’t load tabs until selected' check box in the Preferences menu), when you upgrade to version 47 and up, your setting will reset to allow only one tab at a time. This keeps Firefox running quickly and smoothly. When you restart Firefox and restore your previous session, Firefox will wait until you select a tab before loading it. When you restart Firefox and restore your previous session, Firefox will wait until you select a tab before loading it. I wanted the same and I found the answer here > :
