Fireworks 8 Serial Number

I can't imagine where you got it. Fireworks is End of Life.  It hasn't been sold or supported since 2012.

Unless you purchased a copy 15 years ago, there is no way to get a valid serial number anymore.

Besides it's unfit for use on most modern computers.  It's too old.

Fireworks alternatives.
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- Illustrator CC: https://www.adobe.com/products/illustrator.html
- Photoshop Elements: https://www.adobe.com/products/photoshop-elements.html
- Photoshop CC: https://www.adobe.com/products/photoshop.html

OTHER TOOLS (raster & vector):
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- Affinity Photo & Affinity Designer: https://affinity.serif.com
- CorelDRAW Graphics Suite: https://www.coreldraw.com/
- GIMP: https://www.gimp.org/
- Gravit Designer: https://www.designer.io/en/
- Inkscape: https://inkscape.org/
- Krita: https://krita.org/
- Paint.NET: https://www.getpaint.net/
- PaintShop Pro: https://www.paintshoppro.com/
- PhotoPea: https://www.photopea.com/
- Photo Pos Pro: https://www.photopos.com/PPP3_BS/Default.aspx
- Pixlr: https://pixlr.com/
- Vctr: https://vectr.com/

Hope that helps.