Using a different browser does not autofill because if no one has visited the form before with that browser. If it is set to autofill, then it remembers what you type in for the next time or for the next person after you to steal your information.
One way to overcome this is to set the browser preference to not autofill. See for example http://mzl.la/LRnhjb to disable on Firefox.
Another option would be to clear the cache and then open a "New Private Window" from the File menu which is not suppose to save or supply stored info to sites.
Security and privacy
"Site policy URL for guests" keeps getting auto filled
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