This is a slightly old question.
But it is actually possible to change the proxies dynamically thru a "hacky way"
I am going to use Selenium JS with Firefox but you can follow thru in the language you want.
Step 1: Visiting "about:config"
driver.get("about:config");
Step 2 : Run script that changes proxy
var setupScript=`var prefs = Components.classes["@mozilla.org/preferences-service;1"]
.getService(Components.interfaces.nsIPrefBranch);
prefs.setIntPref("network.proxy.type", 1);
prefs.setCharPref("network.proxy.http", "${proxyUsed.host}");
prefs.setIntPref("network.proxy.http_port", "${proxyUsed.port}");
prefs.setCharPref("network.proxy.ssl", "${proxyUsed.host}");
prefs.setIntPref("network.proxy.ssl_port", "${proxyUsed.port}");
prefs.setCharPref("network.proxy.ftp", "${proxyUsed.host}");
prefs.setIntPref("network.proxy.ftp_port", "${proxyUsed.port}");
`;
//running script below
driver.executeScript(setupScript);
//sleep for 1 sec
driver.sleep(1000);
Where use ${abcd} is where you put your variables, in the above example I am using ES6 which handles concatenation as shown, you can use other concatenation methods of your choice , depending on your language.(The SetupScript is a string containing the script to be runned enclosed by ``)
Step 3: : Visit your site
driver.get("https://whatismyip.com");
Explanation:the above code takes advantage of Firefox's API to change the preferences using JavaScript code.