From bug 1500326 Comment 9, originally reported by James Abbatiello:

I've retested in Firefox Developer Edition 67.0b3 / Win32. I'm seeing some strange behavior.

I start by opening http://example.net/ in a new tab. This performs the 302 redirect to http://example.com/ and loads that content into the window. But the URL bar incorrectly says "example.net".

If it was just displaying incorrectly then clicking the button should perform a POST to http://example.com/form. Instead it performs a POST to http://example.net/form which indicates to me that Firefox mistakenly thinks that the content that it has came from example.net instead of example.com where it actually came from.

Clicking the link to "http://example.net" also has some odd behavior. Clicking on it when the URL bar says "example.net" will redirect you and the URL bar will then say "example.com". Clicking on it again will then change the URL bar back to "example.net" which is incorrect. You can toggle back and forth between these two states by repeatedly clicking the link.

So the original behavior of GET requests is fixed but things still aren't working properly. Do you want to keep this open to track that or would you prefer a new bug?