If a fire starts on my property and spreads, am I liable?
You can be, for the damage it causes and potentially for the cost of fighting it. There is no natural ceiling on that figure, which makes it the largest liability exposure in forest country and the clearest case for an umbrella with real limits behind it.
Even if it was an accident?
Liability generally turns on negligence rather than intent, and burning in breach of a restriction, or using equipment without a spark arrestor, is much harder to defend. Accidental does not mean blameless, and the consequences do not scale down to match the intention.
How much umbrella cover is sensible here?
More than in a suburb. The exposure is unusual because the potential loss is not bounded by what you own. We will work through a figure with you against the realistic reach of a fire from your property rather than quoting a default million.