Allow Firefox to load multiple tabs in the background
When you restart Firefox and restore your previous session, Firefox will wait until you select a tab before loading it. This keeps Firefox running quickly and smoothly.
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 PreferencesOptions menu), when you upgrade to version 47 and up, your setting will reset to allow only one tab at a time.
You can change it back through your about:config preferences (advanced users only):
In the address bar, type about:config and press EnterReturn.
- The about:config "This might void your warranty!" warning page may appear. Click to continue to the about:config page.
- Search for browser.sessionstore.restore_on_demand.
- Change the value to false.
- Close the about:config tab to save your changes.