There's a couple of old Bugs (over 8 years old) in Firefox that can cause that to happen with some web pages. One Bug was recently fixed for Firefox 4.0b8, but it doesn't look like it will be applied to Firefox 3.6.x versions.

There may be a workaround you could apply for a specific domain via a Stylish script, but we'd need the URL of that page and a screen shot of the Print Preview window to try to help you fix it.

Just tried Print Preview on this answer page and got five pages of info over and above the web page's actual one page content. Truly bizarre!

This forum page doesn't use special media = print css coding, so it prints the top and bottom navigation hyperlinks which take up almost 2 of those 5 pages you see in Print Preview.