You will need to use a video editor (Premier, Premier Elements, possibly even Adobe Express) to stitch videos together.
Using the Firefly model, you can only generate 5 seconds at a time. To generate a longer video, you can take the final frame of a generation and use it as the starting keyframe for the next generation. Then assemble the multiple 5 second videos together into one longer video in the video editor.
My best,
droopy