Firefox 52.9.1 ESR has actually been released here, but apparently, it has not made it into PortableApps yet:
As far as we know, this is the latest single-process-capable Firefox version released, and has many security patches that have been backported.
The latest PortableApp that has been released is 52.9.0 ESR released here:
So, we kindly request that 52.9.1 ESR be released as a PortableApp.
As a side note, if anyone knows if 52.9 ESR (AKA single-process Firefox) is continuing to be developed anywhere (even if not by Mozilla), please let us know.