She discovered the incendiary device only when she opened one of the side pockets for the first time on Boxing Day.
The 24-year-old woman dropped the canvas hold-all and called the police.
Officers quickly arrived at her Isle of Wight home, evacuated the house and deactivated the device.
Police believe the firebomb was one of a series planted by animal rights activists in high street shops on the island in August this year when fires in four stores caused damage estimated at £2m.
Inspector Steven Evans said yesterday: "This was the bomb that got away. Because it did not go off at the time in August no-one knew it was there. The woman bought the bag on 21 October. For the next two months she was unwittingly walking around with thedevice. She used the bag on several occasions.
"If it had gone off, the firebomb could have caused untold damage to her house and she could have been very badly injured."
He added: "The woman was unpacking in her bedroom after going away and was taking things out of the main bag. She decided to have a look in the side pocket for the first time and it was then that she found the bomb, which was about the size of a cassette.
"She is a very lucky lady and is obviously shocked by what has happened.
Dozens of people were evacuated from stores during the bombing campaign in August.Reuse content