Could you explain the context of the statement? The D2X firmware was written specifically for the D2X and the D300's firmware was written specifically for the D300. Think of Firmware as the cameras operating system.
Im not sure what it means either. There is a D300 for sale on this website in the "I want to sell" forum, and one of the description items is that it has the "D2x firmware". I don't have any idea about this firmware and was just wondering what it would do for a D300.
It sounds like the seller has confused a firmware update with loading Picture Controls.
The D300 comes standard with 4 picture controls, Standard, Neutral, Vivid and Monochrome. A total of 10 picture controls can be present in the picture control menu. Nikon has 3 downloadable D2X picture controls which can be loaded into the D300 (also in the D3, D3X, and D700).
Additional picture controls from other sources can also be loaded in the D300.
In addition to the 4 standard controls, I've also loaded D2XMode1, D2xMode1_2, Landscape, Fuji Velvia, Fuji Astia and Agfa Optima into my D300. I don't remember where I got the film controls as it was a few years ago that I loaded them.