No, it means that the cause of the fire was accidental. As in the adjective form of accident - something that happens unexpectedly and is not planned in advance.
In the real world of accident investigation—the door to which seems permanently closed in Vixen's mind—these designations are tremendously important.
Very rarely we see evidence consistent with a deliberate act. In most cases we stop and preserve the evidentiary record for regulatory or law enforcement activity.
"Accidental" is generally reserved for cases of pure probability, but we cannot draw a bright line. In practice this is what the late, great Charles Perrow would have called a "normal accident." By this we mean the behavior of complexly organized systems with varying degrees of coupling that result in undesired outcome despite our best efforts to govern the system. This includes unexpected interactions, the intrusion of natural phenomenon, but excludes negligence and malfeasance by unexpected actors (e.g., sabotage).
When we say a system is complexly organized, we mean that causal chains aren't unary or unidirectional. Any given observable quantity in the system may have several possible causes, several effects of interest, and may participate in feedback loops that make causal determinations ahead of time (i.e., during the engineering design process) quite difficult to nail down and model accurately. Structure fires with certain combustible loads are a reasonably good example of a complex system.
When we say a system is coupled tightly or loosely, we mean that cause and effect are separated in time or space to varying degrees. Rocketry is tightly coupled: when things go wrong, they generally proceed from cause to effect in small fractions of a second. Structure fires and survivability should
not be tightly coupled. We prefer loosely coupled systems, and good system design features artificial decouplings.
"Negligent" describes operator's behavior that can be clearly seen as a breach of duty. It doesn't cross the line into deliberate acts. Instead it describes the failure to perform some action that was reasonable and expected, and would have probably interrupted the causal chain leading to an accident. The failure on the part of an individual vehicle owner to maintain his vehicle to the degree it doesn't catch fire could be considered negligence. Or also the failure of regulatory bodies to make and enforce maintenance requirements. But these specific examples are academic determinations. They form part of a cost-risk-benefit analysis.
"Complacency" comprises a gray area between negligence and accident: it's having one foot on the dock and one foot on the boat, in terms of causality analysis. For investigative purposes we tend to resolve those on the side of negligence if there is an overt, persistent, or egregious omission. But we're conscious of the normal behavior of humans to become complacent until failure actually occurs. And we're conscious of the pressure sometimes put on system operators by owners and regulators who assume either that the system is simple enough to operate safely under all circumstances, or that operators have superhuman capabilities to thwart all failure. So if a common level of complacency seems to be the proximate cause, we'll term it an accident. This doesn't release the operator from liability or responsibility in all cases and for all purposes, but it's usually not our job to apportion actionable blame.
So yes, when we speak of "accidental ignition," this has a specific meaning. We may posthumously be able to identify individual points of negligence or complacency. But that doesn't preclude us from being able to say that the opposite of ignition arising from accident is deliberate ignition. No evidence so far of that in this case.