CORS preflight channel did not succeed

I'm trying to build a Ember app with PHP REST framework as my api locally. The Ember app is being served at http://localhost:4200 and the api is being served from just http://localhost. This is causing a CORS issue. I've tried everything that I can think of, but I keep getting an error back saying the request was blocked and that the preflight channel did not succeed. It doesn't succeed in Firefox or Chrome.

I've added the following to the .htaccess file for my api:

Header set Access-Control-Allow-Methods "GET, POST, OPTIONS"
Header set Access-Control-Allow-Origin "http://localhost:4200"
Header set Access-Control-Allow-Credentials true
Header set Access-Control-Allow-Headers "accept, content-type"

Here's my request headers:

Host: localhost
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:39.0) Gecko/20100101 Firefox/39.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Origin: http://localhost:4200
Access-Control-Request-Method: POST
Access-Control-Request-Headers: content-type
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache

And the response headers:

Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: accept, content-type
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Origin: http://localhost:4200
Connection: close
Content-Type: text/html; charset=utf-8
Date: Fri, 24 Jul 2015 17:10:49 GMT
Server: Apache/2.4.9 (Win64) PHP/5.5.12
Set-Cookie: 24fd751c8630b64fcf935a94e8bcef46=qih6pfnqo94d4cgi5b5d79h4i6; path=/
Transfer-Encoding: chunked
X-Powered-By: PHP/5.5.12
p3p: CP="NOI ADM DEV PSAi COM NAV OUR OTRo STP IND DEM"

Any ideas or solutions? Any help is appreciated. Thanks!