To reproduce the bug, open your Firefox Developer Tools Network tab, and then click the button below. Your HTTP request method should then change to POST (it was just "GET".) Once you've done that, right click on the Network Tab line item representing your POST request to this page, and choose the context menu item "Open in New Tab."

If your new tab says your request method was GET, then the request represented by the Network Tab was not reissued and the response displayed in the new tab. Instead, Firefox issued a GET request.

Your HTTP request method was: GET.

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