Could you double-check your home page setting to make sure this isn't set up there? Firefox can be set up with multiple "home tabs" that appear at startup and in any new window (Ctrl+n). To edit your home tabs, you can use the Options page.

"3-bar" menu button (or Tools menu) > Options

Toward the top of the page, in the Startup section, check the Home Page field:

Look for a vertical bar character. As a simple example with short URLs:

https://www.google.com/|https://outlook.live.com/owa/

The bar definitely can be hard to spot with long URLs, but when you find it, delete everything to the right of it then, finally, the bar.

Firefox will save the edit when you tab out of or click away from that box. Then you can close the tab and Firefox should not open it along with your home page.

You can test by pressing Ctrl+n to launch a new window. Success?

See also: How to set the home page

If that is not the source of the problem, could you try disabling or removing the extension named "Firefox OnBoard"? You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then cast a critical eye over the list on the right side. All extensions listed here are optional. Is that one listed? If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

Add this rule to your mozilla.cfg file and the Privacy Notice tab will not open on first run. Very useful in enterprise environments:

// Disable Policy Notice on First Run pref("toolkit.telemetry.reportingpolicy.firstRun", false);