Use proxy detection code path only on Firefox 79 and less

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/911

This was fixed in Firefox 80:
- https://bugzilla.mozilla.org/show_bug.cgi?id=1618271

platform/firefox/vapi-webrequest.js

@@ -125,6 +125,7 @@
             this.cnameFlushTime = Date.now() + this.cnameMaxTTL * 60000;
             // https://github.com/uBlockOrigin/uBlock-issues/issues/911
             //   Install/remove proxy detector.
+            if ( vAPI.webextFlavor.major < 80 ) {
                 const wrohr = browser.webRequest.onHeadersReceived;
                 if ( cnameUncloak === false || cnameUncloakProxied ) {
                     if ( wrohr.hasListener(proxyDetector) ) {
@@ -139,6 +140,7 @@
                 }
                 proxyDetectorTryCount = 32;
             }
+        }
         normalizeDetails(details) {
             if ( mustPunycode && !reAsciiHostname.test(details.url) ) {
                 parsedURL.href = details.url;