Is the yellow float in ford E-150 transmission loose or is it attached to something?

The "yellow float" you're referring to is likely part of the transmission's fluid level indicator. In a Ford E-150, this isn't a freely floating object; it's attached to a mechanism within the transmission's dipstick tube or a similar arrangement. It's designed to move up and down with the fluid level, indicating whether the transmission is properly filled. It's not loose and shouldn't be.