Mark Gilliam, 19, was accused of setting fire to a ceiling tile in the men's bathroom of the Happy Days diner in Thorndale back on January 25, 2009.
Under the terms of a plea agreement Gilliam, of West Bradford Township, will spend another 16 months in prison.
He has been behind bars since February, 2009.
The agreement bars Gilliam from ever serving as a firefighter, even as a volunteer.