I can find no such option in Firefox 2.0.0.1 on Mac OS X. What operating system are you running?
Windows XP, sorry for being a bit autistic - I have no other OS with a gui running here and assumed Firefox would have the same features on every platform.
If it helps, I think the problem that he's describing can be shown with these two annotated screenshots: http://files.nickj.org/MediaWiki/Print-preview-30-percent-scaling.png http://files.nickj.org/MediaWiki/print-preview-200-percent-scaling.png
All the best, Nick.