Shipping a 32-bit browser requires much more than distributing a different build. You also need 32-bit versions of all of the browser's runtime dependencies.
On my box, I see at least the following dependencies for Firefox 3:
linux-vdso.so.1 => (0x00007fff2bbfe000)
/lib/libc.so.6 (0x00007fd922a80000) /lib64/ld-linux-x86-64.so.2 (0x00007fd92399c000)
So, you'd need 32-bit builds of pthread, libc, libgcc, libstdc++, etc., which implies pretty much a complete 32-bit runtime. I also suspect that GLib and Gtk+ should be in that list, which would further bloat the redundant set of libraries.
After a certain point, you should probably just run the browser inside a 32-bit virtual machine, hosted on a 64-bit OS, since you're already going to have to distribute about half a complete distro in runtime requirements.