The problem seems to be related to using file://. If possible you should also host log_in.html on a server.
Bug 1629441 Comment 1 Edit History
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Thank you for your detailed bug report. The problem seems to be that trying to POST a form in an HTML file loaded via file:// to a server using an http:// scheme. Instead of using POST it uses GET. I created a simple testcase to reproduce this locally and arrived at bug 1603006. If possible you could also host log_in.html on a server and it should work.