I'd say that it's unclear whether the events of FC are:
A) a predestination paradox
B) events that all occur within the same timeline
C) events that occur across multiple timelines
Actually it's spelled out explicitly in canon. From "Relativity" ...
DUCANE: The Pogo Paradox.
SEVEN: A causality loop in which interference to prevent an event actually triggers the same event.
DUCANE: Excellent. Can you give me an example?
SEVEN: The Borg once travelled back in time to stop Zefram Cochrane from breaking the warp barrier. They succeeded, but that in turn led the Starship Enterprise to intervene. They assisted Cochrane with the flight the Borg was trying to prevent. Causal loop complete.
DUCANE: So, in a way, the Federation owes its existence to the Borg.
SEVEN: You're welcome.
First Contact, Enterprise, TNG, DS9 and Voyager all occur in the same reality. Anything else is just a fanon conspiracy theory.