Rules 21.6 in the Modules rules says that
Whenever Flames are in play, the rules for them are presented in the scenario's Special Scenario Rules so I'd go with the SSR.
Note that the counters in Not One Step Back (NOSB) 1st edition and NOSB Digital Edition (NOSBDD), ancestors of Heroes of the Motherland (HOTM), were 1-4 Out, 5-6 Burn. The counters in in HOTM 3" box are indeed 1-4 Burn, 5-6 Out. Not sure about NOSB 2n edition of HOTM 1" box. Could anybody owning these last two check for us?
The scenario card in NOSBDD indicate 1-4 Out, 5-6 Burn in the SSR, jsut like in HOTM3. Don't know for the other editions. Could anybody owning them check for us?
I'll admit that 1-4 Burn, 5-6 Out makes for more interesting games
Last time I played Rattenkrieg, the fire fizzled out rather quicly