Which company has used the ad slogan, Forever New Frontiers?

Boeing is the right answer. Boeing launched its slogan Forever New Frontier in 2001 to promote the company after losing its lead in the aircraft business in the previous years. The company was founded on July 15, 1916, by William Boeing in Washington. William bought a shipping yard from Heath that was located by the Duwamish River in Seattle where he transformed the place into his first airplane factory.

Boeing planes were used for airmails across the United States and military aircraft and jets during the second world war and even in the Vietnam war. The company resurfaced after the wars as commercial planes and provided a large seating capacity that no other airlines could not top off. As of now, Boeing supplies commercial planes to significant airlines across the globe.