A Firefly-class ship is also visible in the BG miniseries.
I think you're right. It's a brief glimpse when Laura Roslin (Mary McDonnell) is sitting in the office of her doctor, right?
FYI, it's in the first 15 minutes of the BG pilot episode. Roslyn is at her doctors office when the camera pans past a window. Like every other outdoor scene in the pre-bombardment planets, the sky is filled with spaceships. If you look closely, for just a moment, you can see a Firefly on final approach for landing.
There was a comment by the animators later that they didn't do this so much as a plug as a timesaver. They created both high detail and low detail models for the scenes (depending on how far they were from the camera). The high res ones that are closer to the camera were very time consuming to make, so the animators reused a number of ships that they'd previously created for other projects. Firefly was one of them.