Firefox webextension with proxy array does not work

I was about to make an add-on for FF, generating an array of proxies, all those after the first one as failover options. After endless debugging, I am frustrated to find that only the first proxy is tried by the browser. Put there a dummy as the first proxy, and the browser will never try the second. Connection is never successful if the first proxy fails.
Here is the code:

browser.proxy.onRequest.addListener(handleP, {urls: ["<all_urls>"]});
function handleP(requestInfo)
return [{type: “socks”, host: “”, port: 1443,proxyDNS:true,failoverTimeout:5},{type: “socks”, host: “”, port: 1443,proxyDNS:true,failoverTimeout:5}];

Please do NOT advise using PacScript mode with the proxy.settings.set API, since Firefox Android does not support it.