The Lassiter was a valuable, antique laser pistol which was the forerunner of modern laser technology. It was in the collection of Durran Haymer when Saffron joined Malcolm Reynolds to carry out her plan to steal it. According to her, it was one of only two known to still exist. The theft was successful, ultimately leaving the Lassiter in the hands of Mal, despite Saffron's attempts to take it for herself. Though it was no longer functional, the Lassiter was still worth a fortune, although it's being so well-known and recognizable made it extremely hard to find a buyer.
Behind the scenes
As described and shown in Still Flying, the prop used for the Lassiter had a storied history in Hollywood productions, going back as far as 1992. It was chosen by the Firefly propmaster from a rental house. The main body of the prop was constructed from an old flashlight, and the handle was apparently a solid aluminum foundry casting of a '60s plastic toy ray-gun. It was treated with effects to make it appear like an artifact for filming.
The prop also appeared in the seaQuest DSV episode "Abalon".
It was named after John Lasseter, a talented friend and collaborator of Joss Whedon.
The Lassiter prop is on display at the MoPop museum in Seattle, Washington.