How do I flip through different Firefox windows (not tabs) using cmd < on mac

On my macbook pro (SL) I still run Firefox 16.0.2 where I am used to be able to flip through different windows (incl the downloader window) using cmd and < (Swedish keyboard) which has been very handy. However on my macpro (ML) I have Firefox version 18.0 and can not use that key command anymore, any ideas as to why that is so, anyone?

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