I should probably get a handle on the differences between the two styles because when I read a question posted about "Fusion", the idea in my head is that the question is referring to what I've always heard described as "Free Form Jazz". Are they one in the same thing? Incidentally I've also heard this style referred to as progressive Jazz, so I wonder if some folks are just making up names for this music as they go along trying to describe it.
Fusion is the fusing of jazz and another genre usually rock or funk. So that may mean jazz played with the timbres/tones of rock/pop like distortion or synthesizers. Or it might mean jazz with a funk rhythm section, etc.
Free form jazz or "free jazz" is jazz that is free from the typical rules like song forms, progression, or anything else that defines a particular sub-genre of jazz. For example to sound like bebop you'd use certain song forms, progressions, rhythms, melodies and improvised lines that match that idiom. Free jazz purposely eschews those kind of rules in favor creativity in the moment whatever that may mean to the person playing.
As a result of the lack of rules, free jazz can often sound dissonant and experimental. Also the fact that it's supposed to be creative, experimental and without rules means that it's harder to define because we typically define genres by such idiomatic rules. The definition is more about the process than the end result.
The overlap and confusion comes in that fusion can (but doesn't have to be) also be dissonant and experimental. The difference is—if only in theory—that fusion specifically tries to borrow from the rules, techniques, and sounds of other genres whereas free jazz insists that the rules don't exist.