Mullet the last I bought was labeled FF Plus, same price as old FF. Kinda like "new, improved" Tide washing poiwders I reckon. As an aside, you'd think as improved as them washin' powders have been, you'd only have to wave the box over yer laundry these days. LOL This was last summer at MoJam (The Nocking Pont). Don't honestly know if they quit making it. $22 for 9000' is a good deal, best in the game in my view, if you make as many strings as selfbowyers are liable to. I like linen for staying natural, but it's just not as durable and is more expensive by a good measure. When you get into the designer synthetics you can pay 2 to 3 times the price. Appropriate maybe for target archers, big budget glass bows, but just doesn't seem like it's worth it for an inherently inefficient wooden bow.
I hate to say it but while B50 is cheap, it ain't hardly fit even for even tillering strings in my view. I ain't trying to pick a fight, but what did you dislike about FF, and prefer about B50?