moves Firefox-specific microsummary directory definitions to nsBrowserDirectoryProvider

bug=337324
r=bsmedberg
a=mconnor for the 1.8 branch

browser/components/microsummaries/src/nsMicrosummaryService.js

@@ -439,8 +439,7 @@ MicrosummaryService.prototype = {
 
     var genName = doc.documentElement.getAttribute("name");
     var fileName = sanitizeName(genName) + ".xml";
-    var file = this._dirs.get("ProfD", Ci.nsIFile);
-    file.append("microsummary-generators");
+    var file = this._dirs.get("UsrMicsumGens", Ci.nsIFile);
     file.append(fileName);
     file.createUnique(Ci.nsIFile.NORMAL_FILE_TYPE, PERMS_FILE);