Description of problem: Installing firefox.i386 without firefox.86_64 is not possible, because yelp.x86_64 has a library requirement for firefox.x86_64. yelp.i386 from the i386 repo resolves the gnome requirement for yelp. Version-Release number of selected component (if applicable): yelp-2.19.1-3.fc8 firefox-2.0.0.5-3.fc8
??? We don't claim to provide the resources to completely replace any bit of x86_64 infrastructure with i386.
firefox.i386 is included in the x86_64 distribution, but it can not be used. Most firefox plugins are i386, therefore it is a problem to use them in the x86_64 version. There is a pluginwrapper for x86_64, I know, but there are plugins, which are not correctly working with it.
Then fix nspluginwrapper; the point of including firefox.i386 is not to allow its use as the sole firefox, much as the point of including libgnomeui is not to allow the use of a 32-bit GNOME stack as the full desktop.