Try go to about:config and search for browser.tabs.loadDivertedInBackground change its value to false.

Verify if this occurs in SafeMode:

• Firefox in safe mode

The "Don't load tabs until selected" setting is for loading normal (not pinned) tabs when you start Firefox and reopen tabs from the previous session.

Is that what you try to do?

I'm not sure I have understand. How this option works? I think that while I'm surfing and I open a link in a new tab, if this option is selected, the new tab won't load until I select it. Am I wrong?

Thank you cor-el

@Diego Victor That option is already set to "false".

Thank you

Yes.
That setting is not labeled very clearly and only has effect in restoring tabs automatically when you start Firefox.
As you can see in the bug report, this setting was originally found under the startup setting.

• Tools > Options > General > Startup: "When Firefox Starts":
  "Show my windows and tabs from last time"

• bug 648683 - Expose tabs on-demand preference

Thank you cor-el. The option works; instead I didn't understand how it works.

IMHO, of course, that setting is not labeled clearly.

having this same issue, however I understand how it works. It works on my main PC but not my other. should be no real difference...even did a fresh install and tabs still loading when starting FF

Note that Firefox restores pinned tabs automatically and that by default only normal tabs are affected.

These related prefs can be found on the about:config page.

• browser.sessionstore.restore_hidden_tabs
• browser.sessionstore.restore_pinned_tabs_on_demand