Nathan Parish had worked as an assistant manager at National Tyres and Autocare but left his job about a fortnight before setting the fire – which he says was accidental, Telford Magistrates Court heard.
The court was told he left his job following a dispute with a colleague.
The blaze damaged outbuildings and Parish was forced to pay £500 in compensation to the business after pleading guilty to arson.
The 26-year-old, who is a serving prisoner at Hewell jail and formerly of Grove Court, Ruyton XI Towns, also admitted using threatening words and behaviour towards a worker at the firm on August 15 last year.
Ms Abigail Hall, prosecuting, said the victim told of an ongoing dispute between the pair that came to a head on that day, when Parish used threatening language towards him.
She said police were called to the business in Whitchurch Road on September 3 after someone heard the sound of breaking glass.
Ms Hall said: "Officers attended and noticed broken windows and an ongoing fire."
She said fire investigation officers suspected it was deliberate and Parish later called 999 and stated he had set the fire.
Mr Brendan Reedy, representing Parish, said when he became assistant manager he felt his manager was trying to undermine him and he had decided to leave.
He said: "In respect of the arson this isn't a revenge matter. "It is minor damage by fire.
"He had been drinking. He went around the back of National Tyres and Autocare. He was having a cigarette. "Around the back were discarded tyres.
"He threw a cigarette away. "When he came back later it was on fire.
"He says it wasn't deliberate on his part."
Mr Reedy said Parish had phoned the police himself and told them he had set the fire.
He added: "After he left this job not surprisingly that caused problems at home. "His relationship split up.
"He started drinking. He was using drugs."
Magistrates sentenced Parish to a six week jail term for the arson and there was no separate penalty for the threatening behaviour offence.
He was also ordered to pay £500 in compensation to the firm once he is released from prison.