--- a/addon-sdk/source/lib/toolkit/loader.js
+++ b/addon-sdk/source/lib/toolkit/loader.js
@@ -27,17 +27,17 @@ const { classes: Cc, Constructor: CC, in
         results: Cr, manager: Cm } = Components;
 const systemPrincipal = CC('@mozilla.org/systemprincipal;1', 'nsIPrincipal')();
 const { loadSubScript } = Cc['@mozilla.org/moz/jssubscript-loader;1'].
                      getService(Ci.mozIJSSubScriptLoader);
 const { addObserver, notifyObservers } = Cc['@mozilla.org/observer-service;1'].
                         getService(Ci.nsIObserverService);
 const { XPCOMUtils } = Cu.import("resource://gre/modules/XPCOMUtils.jsm", {});
 const { NetUtil } = Cu.import("resource://gre/modules/NetUtil.jsm", {});
-const { join: pathJoin, normalize, dirname } = Cu.import("resource://gre/modules/osfile/ospath_unix.jsm");
+const { normalize, dirname } = Cu.import("resource://gre/modules/osfile/ospath_unix.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(this, "resProto",
                                    "@mozilla.org/network/protocol;1?name=resource",
                                    "nsIResProtocolHandler");
 XPCOMUtils.defineLazyServiceGetter(this, "zipCache",
                                    "@mozilla.org/libjar/zip-reader-cache;1",
                                    "nsIZipReaderCache");
 
@@ -383,20 +383,20 @@ function readURI(uri) {
 }
 
 // Combines all arguments into a resolved, normalized path
 function join(base, ...paths) {
   // If this is an absolute URL, we need to normalize only the path portion,
   // or we wind up stripping too many slashes and producing invalid URLs.
   let match = /^((?:resource|file|chrome)\:\/\/[^\/]*|jar:[^!]+!)(.*)/.exec(base);
   if (match) {
-    return match[1] + normalize(pathJoin(match[2], ...paths));
+    return match[1] + normalize([match[2], ...paths].join("/"));
   }
 
-  return normalize(pathJoin(base, ...paths));
+  return normalize([base, ...paths].join("/"));
 }
 Loader.join = join;
 
 // Function takes set of options and returns a JS sandbox. Function may be
 // passed set of options:
 //  - `name`: A string value which identifies the sandbox in about:memory. Will
 //    throw exception if omitted.
 // - `principal`: String URI or `nsIPrincipal` for the sandbox. Defaults to
@@ -594,17 +594,21 @@ function normalizeExt(uri) {
 // `requirer.uri` but in some cases it may be `baseURI`. In order to
 // avoid complexity we require `baseURI` with a trailing `/`.
 const resolve = iced(function resolve(id, base) {
   if (!isRelative(id))
     return id;
 
   let baseDir = dirname(base);
 
-  let resolved = join(baseDir, id);
+  let resolved;
+  if (baseDir.includes(":"))
+    resolved = join(baseDir, id);
+  else
+    resolved = normalize(`${baseDir}/${id}`);
 
   // Joining and normalizing removes the './' from relative files.
   // We need to ensure the resolution still has the root
   if (base.startsWith('./'))
     resolved = './' + resolved;
 
   return resolved;
 });
@@ -657,17 +661,17 @@ function resolveRelative(rootURI, module
 // From `resolve` module
 // https://github.com/substack/node-resolve/blob/master/lib/node-modules-paths.js
 function* getNodeModulePaths(rootURI, start) {
   let moduleDir = 'node_modules';
 
   let parts = start.split('/');
   while (parts.length) {
     let leaf = parts.pop();
-    let path = join(...parts, leaf, moduleDir);
+    let path = [...parts, leaf, moduleDir].join("/");
     if (leaf !== moduleDir && urlCache.exists(join(rootURI, path))) {
       yield path;
     }
   }
 
   if (urlCache.exists(join(rootURI, moduleDir))) {
     yield moduleDir;
   }