Don't use io.newChannelFromURIWithLoadInfo(…, null) in Firefox 37-43

Also see https://bugzilla.mozilla.org/show_bug.cgi?id=1182571
(closes #247)

protocol.jsm

@@ -86,11 +86,12 @@ var privateProtocol = {
 		// Also we can't use nsIPrivateBrowsingChannel.setPrivate(true) for chrome:// URI
 		var redirect = "chrome://privatetab/content/protocolRedirect.html#" + newSpec;
 		var channel = "newChannelFromURIWithLoadInfo" in Services.io // Firefox 37+
+			&& (loadInfo || parseFloat(Services.appinfo.platformVersion) >= 44) // Throws in Firefox 37-43 with null nsILoadInfo
 			? Services.io.newChannelFromURIWithLoadInfo(
 				Services.io.newURI(redirect, null, null),
 				loadInfo
 			)
-			: Services.io.newChannel(redirect, null, null); // Removed in Firefox 48+
+			: Services.io.newChannel(redirect, null, null); // Deprecated in Firefox 48+
 		var ensurePrivate = function(reason) {
 			_log(reason + " => ensurePrivate()");
 			this.makeChannelPrivate(channel);

protocolRedirect.html

@@ -28,6 +28,7 @@
 	// Validate URI
 	var uri = Services.io.newURI(spec, null, null);
 	var testChannel = "newChannelFromURIWithLoadInfo" in Services.io // Firefox 37+
+		&& parseFloat(Services.appinfo.platformVersion) >= 44 // Throws in Firefox 37-43 with null nsILoadInfo
 		? Services.io.newChannelFromURIWithLoadInfo(uri, null)
 		: Services.io.newChannelFromURI(uri); // Deprecated in Firefox 48+
 	document.title = getLoadingTitle() || spec;