Just look at the request type from both pictures. The one in Chrome is a POST request while the one in FireFox is a GET request. I assume there’s something wrong in the code or something was redacted from the posted code.
Server-side scripting like php is browser agnostic, so it should not make any difference.
This points to a problem on the client side, likely the html of the form.
Best to check for (post) form submission like this:-
if($_SERVER['REQUEST_METHOD'] === 'POST'){
// form processing here
}