No, Einstein's special theory of relativity is not based on the concept of an ether. In fact, one of the key features of special relativity is the abandonment of the concept of a luminiferous ether, which was originally proposed in attempts to explain the propagation of light.
The luminiferous ether was a hypothetical medium that was thought to fill all of space and serve as a reference frame for the transmission of light waves. According to this view, light waves were believed to propagate through the ether in a manner similar to how mechanical waves propagate through a medium like water or air.
However, experiments such as the Michelson-Morley experiment in the late 19th century failed to detect any evidence of the existence of the ether. These experiments showed that the speed of light was constant and independent of the motion of the observer, which contradicted the expected behavior if there were an ether.
Einstein's special theory of relativity, published in 1905, introduced a new understanding of space, time, and the nature of light. It proposed that the laws of physics, including the behavior of light, are the same in all inertial reference frames. Special relativity replaced the concept of an ether with the idea that the speed of light in a vacuum is constant for all observers, regardless of their relative motion.
By discarding the need for an ether and introducing the principles of relativity, Einstein's theory provided a more coherent and consistent framework for understanding the behavior of light and the nature of space and time.