Under Fireworks > Preferences > General there is an option for Bicubic and other interpolation methods. Is this what you're looking for? I have the latest version of Fireworks. Not sure what particular version you have and if it offers those options.
I have those options in Fireworks 8, too. They affect the interpolation used for the Scale tool and the Transform commands. They even impact Modify > Canvas > Image Size... which is odd because there's a separate dropdown menu for that within the command dialog. (However, they don't affect resizing within the Image Preview dialog, which appears to be bicubic.)
Unfortunately, these preferences don't affect the rendering behavior of the zoom tool. At view percentages less than 100%, things look pretty rough.