Vertical lines in the lower half of the screen appear while the screensaver is running. I think there is a memory corruption here, not the graphical glitches because the problem happens with the simulation of the fire, not with rendering. It seems like these lines somehow appear in the fire buffer (this might be true only if this screensaver implements a classical demo fire effect) and then this corrupted fire buffer is rendered to the screen.
Free Fire Screesaver by Laconics Software.
NOTE: The license of the screensaver says (24 JAN 2013): "You may not decompile, reverse engineer, disassemble or otherwise reduce this software to a human perceivable form."