I can replicate these failures under Windows XP using Firefox 10.0.1 as well. It seems that Mozilla broke backwards compatibility there. If you only care about using WWW::Mechanize::Firefox, you should be able to force-install MozRepl, as MozRepl::RemoteObject uses a different method of transferring (resp. delimiting) data. At least, the test suite for both modules still pass even though the test suite for MozRepl fails.
Update: It seems that most (but only) MozRepl plugins are affected, like the JSON plug-in. I haven't looked at what could be the cause though.