I'm now testing against Firefox 8 as well, because now there are portable versions of it. At least using the latest version of mozrepl at commmit 552ed9b1, or maybe just the latest as a zip file, tests pass for MozRepl::RemoteObject 0.28.

If you are trying to install "the latest" mozrepl from a zip file, you need to restructure the zip file to remove the top level directory, and then rename the zip file to .xpi for easy installing.