Started a new world in latest snapshot. Lava spread to grass and then went to trees (normal) but didn't stop. When I came back to the area a while later, a larger-than-usual area was on fire. RandomTickSpeed command made it stop spreading when 0, however changing it to 1000 didn't seem to change anything, probably because the fire was already spreading that fast. doFireTick stopped the fire as well. Memory was high because of fire. Photo of situation is attached. Was not tampering with RandomTickSpeed command until after I found the fire and I was in Singleplayer. In photo, lavafall was source of fire, and as you can see it spread without dying out. Fire continued to another forest out of sight.