Why is my furnace leaking, and what is it leaking?
I know it's in the comments, but I feel it's important enough to place in an answer:
Never EVER throw water on a fire that is , or might be, oil or gasoline - based!
If your furnace does not have an automatic thermal cutoff valve on the fuel line inlet, you're out of code (at least in Massachusetts), and should definitely have a plumber install one.
Keep a fire extinguisher that's rated for gas/oil/volatiles near your furnace; but if a fire starts, always call the fire dep't.  They know how to check for hidden &quot;hot spots&quot; .