Tor opens its own socks5 proxy on localhost, so you cannot put in your browser both.
To use your normal HTTP/HTTPS proxy settings you should 'torify' your browser from outside, means route all it's traffic transparently to Tor. I am not sure this could be easily done on Windows, but there are many such kind of programs: SocksCap, FreeCap, WinPCap, .. You can google which one of them works with Firefox.
Another way to 'torify' browser is to use VirtualBox setup like http://www.whonix.org
. They have Debian Linux distribution in a virtual box image, with mail, browser and everything inside. So you can use their browser (which is also Firefox) as "normally" (means put there your HTTP/S proxy settings) and also be sure all traffic that leaves this Virtual Machine is first routed to Tor.
Browser -> Tor -> Tor Exit Node -> HTTP proxy -> destination site.
Third way is to have a router such as Raspberry Onion Pi Tor, or to route it manually via route table