As the price of rubber slips on the global market, Firestone — a company founded in Ohio with nine decades in this West African country — is shedding large swaths of its staff to cope with what it calls “continued and unsustainable losses.” The drawdown threatens to rip a seismic hole in Liberia’s floundering economy, analysts say, opening the latest chapter in the country’s long and complicated history with American-rooted power brokers.