WDB ACFT HAS SMOKE IN COCKPIT AND FIRE. REQUESTS NEAREST AIRFIELD AND DIVERTS; DECLARING EMER.

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WE SMELLED SOMETHING ELECTRICAL; SMOKE POURED FROM BEHIND THE GLARESHIELD; ACCOMPANIED BY A HOT; ARC-WELD LOOKING HEAT SOURCE (FIRE). WE DONNED OUR OXYGEN MASKS AND GOGGLES; INQUIRED FROM MONCTON CTR WHERE THE NEAREST AIRFIELD WAS; AND STARTED AN EMER DSCNT; TURNING BACK 180 DEGS TOWARD THAT FIELD. SHORTLY AFTER THE INITIATION OF THE DSCNT WE RECEIVED A WARNING MESSAGE THAT WE HAD AN OVERHEAT IN OUR ELECTRONICS COMPARTMENT (LOCATED BELOW THE COCKPIT FLOOR). COORD WAS EXECUTED WITH THE FLT ATTENDANTS OF OUR SIT AND TIME REMAINING BEFORE WE LANDED. FURTHER INTO THE DSCNT AND AFTER COMPLETION OF THE APPROPRIATE CHKLISTS; THE WARNING LIGHT EXTINGUISHED AND THE SMOKE BEGAN TO DISSIPATE. WE INFORMED THE CABIN CREW THAT WE DID NOT PLAN AN EVAC OF THE ACFT UPON LNDG. A VISUAL APCH WAS MADE TO SYDNEY FIELD YQY AND AN OVERWT LNDG RESULTED. (TOUCHDOWN SINK RATE BTWN 500-700 FPM.) ON ROLLOUT; LCL CFR UNITS CONFIRMED THERE WAS NO FIRE SO WE TAXIED TO THE RAMP AND DEPLANED THE PAX THROUGH THE FORWARD ENTRY DOOR.