How are you even using portable FF with Selenium RC?
In RC, I believe Selenium looks for the (default) FF install location for where to invoke the FF binary. I'm assuming you are talking about setting (*nix/Windows/Mac) environment variable or Windows registry setting that points to FF binary (creating it if not exist, which it likely would not exist for FF portable since no install occurred)?
For RC, you specify Firefox binary when you specify the browser string, and instead of '*firefox' it would be like '*firefox C:\pathTo\firefox.exe' and similarly for Linux/Mac, '*firefox /pathTo/firefox-bin', where for Mac it looks more like '*firefox /pathTo/Firefox.app/morePaths/firefox-bin'
If you were able to use and specify FF binary with RC, then did you try specifying absolute path to FF profile to load with? That might work?
java -jar selenium-server.jar -firefoxProfileTemplate "C:\PathTo\FFProfileDirectory"
I was able to run automation specifying path to portable FF but on Mac. Just recently trying for Windows, not sure if I can get it to work. Haven't tried for WebDriver yet.