Maybe I'm mistaken, but this is just when you scroll, release your fingers, and it slows to a stop instead of stopping instantly, right? I have that out of the box here on Linux. Firefox + X + touchpad with the synaptics driver.

I had thought this was what "Use smooth scrolling" did it in the Firefox preferences.

Edit: I also just checked and I have this in my terminal (Konsole), text editor (KWrite), and PDF viewer (Okular) so this is at the very least not a Firefox only thing. It feels exactly the same in each application. I bet it's a feature of the touchpad driver.