Is the "X" on a bar that pushes down between the toolbar area and the top of a blank page? Sometimes it will refer to blocked popups or ask permission to redirect?

You can manage those features as follows:

(1) Interact with the bar on a one-time basis

(2) Turn off the relevant blocker

The Pop-up blocker is managed from the Privacy & Security section of the Options page.

The redirect block is more obscure:

(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(B) In the search box above the list, type or paste access and pause while the list is filtered

(C) TO TURN OFF: If the accessibility.blockautorefresh preference is bolded and "modified" or "user set" to true, double-click it to restore the default value of false

Does that fix it?

If not, could you give more details about any text or images near the X?

https://screenshots.firefox.com/dg8AQjg2vxPBJIEg/www.yahoo.com

That's interesting. Do you use an ad blocker? Sometimes they will remove the contents of an overlay but not the overlay itself. The "X" likely is a button to close the overlay. If you click it, is the content behind it, or was the content supposed to be in the overlay?