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If you don't enter the extension on firefox (.com, .ca, whatever), it will search for the closest relating website by keywords. So I guess walgreens is the top result for drug store.
I am not sure if they use google for this, but it is similar to Google's "I'm Feeling Lucky" feature.
You can try it with any keyword, the top site with that keyword will always show up.
Example: If you enter "baseball", mlb.com will show up.