There is no Beta builds of ESR versions. There will be Release Candidates of 45.0 ESR when it is close to Release as it will be based on Fx 45.0 Release. The Firefox 44.0 Release just came out yesterday. Beta refers to one of four main development channels Release < Beta < Aurora (developers edition) < Nightly. There is also the separate ESR channel. There will be Beta builds of 45.0 soon and there can be say six to twelve beta builds for a version.