command timed-out at ...
[download]

most likely means that there was some error in the communication between Firefox and MozRepl::Client. You can switch the logging to debug to see what the offending command is. So far, I have not tested WWW::Mechanize::Firefox against Firefox 7, so I don't know what might go wrong.

Does the MozRepl::RemoteObject testsuite pass? Do the WWW::Mechanize::Firefox tests pass?