tinder-box

A small container containing flint, steel, and tinder (dry, finely-divided fibrous matter), once used to help kindle a fire.

“"Just bring me the old tinderbox that my grandmother forgot the last time she went down there."”

(by extension) a place that is so dry and hot that there is danger of fire.

“"Think of your blank journal as a tinderbox, a box for holding combustible materials, ready to catch fire when you sit down to work."”

(by extension) a potentially dangerous situation.

“"This act was the "match that ignited the great tinderbox of fuel" that had been building for years between many of the countries in Europe."”