well, in firefox I think you can empty the parameter general.useragent.locale and the sites you are entering will not know where you are from (never tested).
to change open a browser tab or window and type in the address bar:
search for the general.useragent.locale string and delete it or set for something you want, don't forget to keep in mind the old value so you can set it back if you need.. after that close firefox, open again, clean all your cookies and see if the site still know where you from, if so maybe the site is watching ip bands to know where you're located...
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