Have you tried dropping the 32-bit version of adobe flash into the plugins folder available inside of the root folder for Firefox 4.0b1? Just download the tarball with the precompiled 32-bit plugin, extract the tarball, and then move the plugin into that plugins folder I mentioned. Since you do have lib32 installed on your 64-bit Kubuntu install, the 32-bit plugin should make use of the lib32 libraries and should work under the 32-bit Firefox beta.