The fire broke out in the manhole at Avenue B and East 12th Street at about 8:30 p.m.
A few minutes later, an electrical panel exploded in the basement of 183 Ave. B, injuring the firefighter.
"We heard a loud pop," said East Village resident Ken Petricig, 40, who was eating at a nearby restaurant.
"We all said, 'What was that?' - like it was a bomb."
The firefighter was taken to Bellevue Hospital. Officials said his injuries did not appear to be serious.
An FDNY source said the explosion occurred when the manhole fire spread to the basement.
But Con Ed spokeswoman D. Joy Faber insisted it wasn't clear if the two incidents were connected.
The cause is under investigation and fire officials are checking to see if it was weather-related.
The fire and explosion occurred about three blocks from where Jodi Lane, 30, was killed by stray voltage while walking her dogs in January.
Earlier this month, two more dogs were shocked while walking on First Avenue near East Ninth Street.
Faber said what happened last night "is an isolated incident. There is no direct correlation at all with the other incidents."
