Names can change over time but here is how i recall it:

FRP is a development based on the work of conal elliott: http://conal.net/ It's main characteristic is that there is a continuous (pull)aspect. Since many interesting domains are not predominantly continuous he and many others had to add a discrete event based component.

Rx is a stream based technology (push). Basically it is a dataflow library with modern functional programming syntactic elements. It does not have a continuous aspect and as such it is not FRP.

By treating Rx as something new we are depriving our self of the dataflow and structured programming knowledge.