Firefox provides me a number of "search boxes" to access websites/apps that I commonly use. How do I add/change new/different sites that I regularly use to this convenient collection of easy options?

Note what I wrote above. I simply want to optimize my use of this option and can't if it doesn't allow me access to sites that I desire to access rather than ones that simply pop into these squares as existing, irrelevant, I suppose Firefox placed, options are deleted. Most of which I've seen and received are irrelevant to me.