Use multiple proxies on Firefox -

Create pac file proxy.pac with content:

function FindProxyForURL(url, host) {
	var proxy1 = "PROXY 1st.proxy.com:port";  // <-- Update here
	var proxy2 = "PROXY 2nd.proxy.com:port";  // <-- Update here
	var no_proxy = "DIRECT";

	var proxy2_hosts = ["*example.com"];            // <-- Update here
	var no_proxy_hosts = ["127.0.0.1","localhost"]; // <-- Update here

	for (i = 0; i < no_proxy_hosts.length; i++) {
		if (shExpMatch(host, no_proxy_hosts[i])) return no_proxy;
	}

	for (i = 0; i < proxy2_hosts.length; i++) {
		if (shExpMatch(host, proxy2_hosts[i])) return proxy2;
	}

	return proxy1;
}

Open Firefox Preferences -> Network Settings.
Check Automatic proxy configuration URL.
Set file path to proxy.pac: file:///path/to/proxy.pac
Reload.
OK.

firefox proxy

See also