Installing Firefox plugins issue

I am trying to install my own firefox plugin. I follow this tutorial. But each time thi开发者_开发知识库s gives me following error:-

Firefox could not install the file at 

file:///home/ubuntu/googbar.xpi

because: Install script not found
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That error means that your file doesn't have an install.rdf file. Please note that it needs to be at the top level of the XPI archive. So maybe you made a mistake packing the file and install.rdf landed in a subdirectory. You can check the list of files using unzip -l googbar.xpi command, it should list install.rdf without a directory name before it.