When you install Firefox and you don't have any history yet, Firefox displays some popular sites as Top Sites. Possibly some of them will stick around longer than you like. To remove them, you can use the "Dismiss" command. See: Customize your Firefox New Tab page.

Does that work for you, or does Facebook come back?

Thank you, jscher2000.

I've had Firefox for years and I was surprised to find this morning that Facebook had mysteriously become one my 'Top Sites'. This is a first and the reason for my question.

I was aware of the 'Dismiss' facility and had removed Facebook that way.

I am however still puzzled as to how Facebook managed to be a Top Site for me when I go out of my way not to use it in any way.