On straight-in for my home airport, about two miles out, smoke started
swirling around in the cockpit.  It wasn't real bad but it was
definitely noticeable by both sight and smell.  In hindsight it was
clearly an electrical fire (smell and where the smoke was coming from
around the panel) but I quickly declared an emergency saying I had a
fire in the engine compartment.  Don't know why I said that other than
that it was what I was the most afraid of.  There was no place to land
short of the runway so I went into high-speed prayer mode, held gear and
flaps until the last moment and landed more or less normally.  Shut down
on the runway and coasted to a turnoff.  Couldn't do the fuel shutoff
quickly so just turned it to in between left and right.  Switches off,
canopy up and told my wife to leave by the nearest exit.  By the time
she was gone, I was noticing that there was no new smoke so I decided to
hunt around for the fire extinguisher (where is was supposed to be but
under my flight bag.)  With still no indication of a continuing fire, I
started turning off switches and, having by this time convinced myself
that the problem was electrical, took the glare shield off to look
around.  Saw nothing at first but one of the fire guys who showed up
right after that found a small blackened spot in the firewall near the
top right.  Sure enough, the alternator wire pass through had burned
through.  The stud and nuts were fused and a small part of the
surrounding e-glass seemed to have burned.

I had two opportunities to catch this early.  The first was that amps
seemed to be running on the high side.  We put this down to having to
charge up the battery because we had had systems on with the engine off
but, in hindsight, it was a clue.  Then about a week ago, I had the same
smell on approach to Boeing.  There was no smoke, it went away after a
couple minutes and I saw nothing when I removed the glareshield and
inspected.  Also no repeat on the return trip.

Turns out that high amperage connections (like all other connections)
can loosen over time.  If they do so, resistance goes up, amperage goes
up to compensate (at least in the case of the alternator wire) arcing
takes place and eventually the whole system burns up.  There is no short
and therefore no cb's pop but, as I discovered, the process can start a
fire.

The firewall connections are hard to see (at least on the cabin side)
but I am definitely adding that to my annual checklist.  If the rest of
you are not sure your connections are as tight as they were when you put
them together, suggest you check.  I'm also finding a new place for my
fire extinguisher.