It's not MozRepl::RemoteObject that's broken but the mozrepl Firefox Add-on. You need to use a checkout from the Github repository (for example 1.0.2). You could also have searched this site - there is at least one other post with the same problem and same solution.

Update: RemoteObject.pm fails in open firefox8.0 is that other post, as found by Super Search

In reply to Re: Firefox 8 breaks MozRepl/RemoteObject.pm by Corion
in thread Firefox 8 breaks MozRepl/RemoteObject.pm by goldmund

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