Prometheus isn't a CFW by itself. It's a patch for 5.50 GEN-D3 final that allows users to run games that need up to OFW 6.31 (Or 6.35 for the unofficial Prometheus-4).
Originally Posted by Worm9100
The last official version that was made was Prometheus-3v4.
So even if you wanted to use it you have to first upgrade your firmware to 5.50 GEN-D3 final.
But beware: If you upgrade your firmware to 5.50 GEN-D3 final from 5.50 GEN-A, you might see all your save data as being "corrupted" (They only appear to your games as being corrupted, but are actually fine). This will probably mean that you will lose all of your old save data.
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit