Export the frames of your animation as single PNG frames (PNG sequence), and import those in a video editor and export as a H264 MP4.
Fireworks command to export images:
If you are not scared of the command line, it is simple to convert the image sequence by using ffmpeg.
Side note: Fireworks is not exactly the best tool to do this type of work. You would be better off to switch to different software next time. It depends a bit on the type of intro and its content which applications would fit the bill.