Today we wanted to get FUNFX tests running by Maven and then it was easier to get things going with JRuby than with Ruby. We found that regular Firewatir for the moment does not support JRuby, but we found a patch by Sai Venkatakrishnan here http://rubyforge.org/pipermail/wtr-development/2008-November/000323.html
This patch worked great for us on Firewatir 1.6.2. Maybe this patch has been entered into the next release candidate, I did not check. But copy the patch into the case statement in firewatir.rb where it checks for java and you are good to go.
Works great for us.
After installing JRuby and FunFX with the gem installer for JRuby and the same with Firewatir. Just run your scripts with jruby instead of ruby.