This happens when you click the "+" button or press Command+t? Or does it happen in a new window (Command+n)?

For a new tab problem, check for a rogue extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Command+Shift+a (Windows: Ctrl+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious? If in doubt, disable.

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

Any improvement?

For a new window problem, check your home page setting. This article will get you to that section of the Preferences page: How to set the home page.

Are we making progress?

I didn't know you could:

For a new tab problem, check for a rogue extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

   Command+Shift+a (Windows: Ctrl+Shift+a)
   "3-bar" menu button (or Tools menu) > Add-ons
   type or paste about:addons in the address bar and press Enter/Return 

When I did, the problem was obvious and with this help it was solved.

BG