Your policy pays first, whoever started it
If a fire spreads from a neighboring property to yours, your own homeowners policy covers your damage. It does not wait for anyone to establish who was responsible. Your insurer may then pursue the neighbor’s insurer, and that happens between companies rather than between neighbors, which is worth something in a small community.
Close spacing raises what a single fire costs
Where structures sit near each other a fire reaches more buildings. Carriers consider spacing and construction in rating, and it is a characteristic of the neighborhood rather than anything you did. It is also an argument for a dwelling limit with no shortfall in it.
The reverse is the uncomfortable half
A fire starting at your property that spreads to a neighbor is your liability, and in dense housing it can reach several buildings. That exposure is bounded by nothing about your own house, which is the clearest reason here to carry liability above the default.