Did you try .

I periodically switch between pools just to distribute hash power. None of the pool "Getting Started" mention fixed diff, but i have used fixed diff with irdpool.ru, ird.cashpool.us and irdpool tk and var diff with the other pools.
. works with irdpool.ru, ird.cashpool.us and irdpool tk

I would have suggested keeping the same format for irdpool.fr just for consistency but "+" works fine too