How do I make Firefox 47 load all my tabs on startup

I wanted the same and I found the answer here >> https://support.mozilla.org/en-US/kb/allow-firefox-load-multiple-tabs-background:

This applies to versions 47 and up.

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 Preferences 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):

1. In the address bar, type about:config and press Enter.

   The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page.

2. Search for browser.sessionstore.restore_on_demand.
3. Change the value to false.
4. Close the about:config tab to save your changes.

Allowing Firefox to load tabs simultaneously can slow down its startup.

For those who don't want to load all the tabs on startup, my solution was to pin the few tabs that I want always available and keep the rest lazy-loaded by default.

setting browser.sessionstore.restore_tabs_lazily and browser.sessionstore.restore_on_demand together did the trick for me.

It won't affect the startup time, as it's first still lazy, but it will refresh the tabs in the background shortly after restart (a couple of them together).

That's exactly what I was looking for. :)

As of Firefox 68, browser.sessionstore.restore_on_demand no longer works. The workaround I found is using an extension that will reload all tabs when clicked, which also loads in any unloaded tabs. This s the one I used: https://addons.mozilla.org/en-US/firefox/addon/reload-all-tabs-in-window/