--- a/toolkit/mozapps/extensions/AddonManagerStartup.cpp
+++ b/toolkit/mozapps/extensions/AddonManagerStartup.cpp
@@ -380,25 +380,29 @@ public:
 private:
   nsString mId;
   InstallLocation& mLocation;
 };
 
 already_AddRefed<nsIFile>
 Addon::FullPath()
 {
-  nsString path = mLocation.Path();
+  nsString path = Path();
 
+  // First check for an absolute path, in case we have a proxy file.
   nsCOMPtr<nsIFile> file;
-  NS_NewLocalFile(path, false, getter_AddRefs(file));
+  if (NS_SUCCEEDED(NS_NewLocalFile(path, false, getter_AddRefs(file)))) {
+    return file.forget();
+  }
+
+  // If not an absolute path, fall back to a relative path from the location.
+  NS_NewLocalFile(mLocation.Path(), false, getter_AddRefs(file));
   MOZ_RELEASE_ASSERT(file);
 
-  path = Path();
   file->AppendRelativePath(path);
-
   return file.forget();
 }
 
 NSLocationType
 Addon::LocationType()
 {
   nsString type = GetString("type", "extension");
   if (type.LowerCaseEqualsLiteral("theme")) {

--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -119,24 +119,30 @@ const nsIFile = Components.Constructor("
  *
  * @param {string} path
  *        The (possibly relative) path of the file.
  * @param {nsIFile} [base]
  *        An optional file to use as a base path if `path` is relative.
  * @returns {nsIFile}
  */
 function getFile(path, base = null) {
-  if (base) {
-    let file = base.clone();
-    try {
-      file.appendRelativePath(path);
-      return file;
-    } catch (e) {}
-  }
-  return new nsIFile(path);
+  // First try for an absolute path, as we get in the case of proxy
+  // files. Ideally we would try a relative path first, but on Windows,
+  // paths which begin with a drive letter are valid as relative paths,
+  // and treated as such.
+  try {
+    return new nsIFile(path);
+  } catch (e) {
+    // Ignore invalid relative paths.
+  }
+
+  // If the path isn't absolute, we must have a base path.
+  let file = base.clone();
+  file.appendRelativePath(path);
+  return file;
 }
 
 /**
  * Returns the modification time of the given file, or 0 if the file
  * does not exist, or cannot be accessed.
  *
  * @param {nsIFile} file
  *        The file to retrieve the modification time for.