"This cosmos was not made by immortal or mortal beings, but always was, is and will be an eternal fire, arising and subsiding in measure."
The presumption that the cosmos was not made by immortal or mortal beings draws out attention to the verb. The cosmos was not "made" at all, so there is no answer to the question who made it.
The cosmos was not made precisely in the sense that it has always been.
So far so good, we might attribute to Heraclitus something akin to the steady-state theory of the cosmos and knock off our exegetical work for the day. But then what are we to do with the final clause? The cosmos continues to exist by "arising and subsiding in measure."
To continue with the categories of contemporary cosmologists, this seems like a "Big Crunch" theory. The universe is a closed one, in that it will not expand forever (that would be an expansion without "measure"!). It will at some point start to fall in upon itself, with all matter ending up in Black Holes and then the black holes attracting one another so every ends up as One Black Hole, one singularity, whence one could then readily imagine the next Big Bang.
That is, the presumption of a Big Crunch tells us in Heraclitean terms not only how the cosmos will subside in measure, but in principle how the next one will arise. The meta-cosmos, consisting of the whole cycle, may well then be seen as an eternal fire.