But what about a person dropping a lit cigarette on a couch (either sleeping or not). A cigarette isn't an open flame so the above doesn't apply. Is there a reasonable expectation that a lit cigarette would cause a couch to catch on fire? If there is, and a jury agreed, then the person who dropped it would be negligent.
But I would expect said person's lawyer to show that a standard cigarette dropped on a couch would not and could not cause a fire. But cigarettes manufactured by company X are treated with chemical Y in order to make them non-standard. And while the client may be partially negligent (for dropping the cigarette or falling asleep), the manufacture is also partially negligent for (knowingly) adding fire sustaining chemicals to an otherwise standard cigarette. If the cigarette manufacturer could prove that a standard cigarette would catch a couch on fire, and the knowledge of this was obvious to a standard person, then the person would be 100% negligent.
When my apartment building nearly caught fire because someone two buildings over fell asleep with a cigarette in hand, killed themselves, and burned down two other apartments, of course people wondered why cigarettes don't burn out on their own when you stop taking drags.