I'm not a regular Fireworks user but I can't see an obvious way to do it either through 'Save As' or 'Export'. It would probably be easier to get the free IrfanView click here then all you need to do is open the jpeg and save as gif.
Surface Pro (2017) vs Surface Pro 4
Where HTML5 is headed next
MacBook Pro v Surface Pro 5
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.