The error should happen in following code

...
 warn sprintf "(Sub)string is [%s]", substr($js,$offset,20);
        die $@
...
[download]

The function is:

# Take a JSON response and convert it to a Perl data structure
sub to_perl {
    my ($self,$js) = @_;
    local $_ = $js;
    #s/^(\.+\>\s*)+//; # remove Mozrepl continuation prompts
    s/^"//;
    s/"$//;
    
    if (/\.+>/) {
        # This should now be eliminated!
        die "Continuation prompt found in [$_]";
    }
    
    #warn $js;
    # reraise JS errors from perspective of caller
    if (/^!!!\s+(.*)$/m) {
        croak "MozRepl::RemoteObject: $1";
    };
    
    if (! /\S/) {
        # We got an empty string back from the REPL ...
        warn "Got empty string from REPL";
        return;
    }

    # In the case that we don't have a unicode string
    # already, decode the string from UTF-8
    $js = decode('UTF-8', $_);
    #warn "[[$_]]";
    my $res;
    local $@;
    my $json = $self->json;
    if (! eval {
        
        $res = $json->decode($js);
        #use Data::Dumper;
        #warn Dumper $res;
        1
    }) {
        my $err = $@;
        my $offset;
        if ($err =~ /character offset (\d+)\b/) {
            $offset = $1
        };
        $offset -= 10;
        $offset = 0 if $offset < 0;
        warn sprintf "(Sub)string is [%s]", substr($js,$offset,20);
        die $@
    };
    $res
};
[download]

As i remember, with previous firefox releases(6.0,5.0,4.0), i encounter this problem occasionally. Now it happens always in firefox 8. How could avoid this problem?