diff --git a/netwerk/mime/nsMIMEHeaderParamImpl.cpp b/netwerk/mime/nsMIMEHeaderParamImpl.cpp
index da976a4b775..4a60e698b91 100644
--- a/netwerk/mime/nsMIMEHeaderParamImpl.cpp
+++ b/netwerk/mime/nsMIMEHeaderParamImpl.cpp
@@ -253,7 +253,7 @@ nsMIMEHeaderParamImpl::GetParameterInternal(const char *aHeaderValue,
       tempStr.StripChars("\r\n");
       *aResult = ToNewCString(tempStr);
       NS_ENSURE_TRUE(*aResult, NS_ERROR_OUT_OF_MEMORY);
-      return NS_OK;
+      // keep going, we may find a RFC 2231 encoded alternative
     }
     // case B, C, and D
     else if (tokenEnd - tokenStart > paramLen &&
@@ -300,7 +300,12 @@ nsMIMEHeaderParamImpl::GetParameterInternal(const char *aHeaderValue,
 
         if (sQuote2 && sQuote2 + 1 < valueEnd)
         {
-          NS_ASSERTION(!*aResult, "This is the 1st line. result buffer should be null.");
+          if (*aResult)
+          {
+            // drop non-2231-encoded value, instead prefer the one using
+            // the RFC2231 encoding
+            nsMemory::Free(*aResult);
+          }
           *aResult = (char *) nsMemory::Alloc(valueEnd - (sQuote2 + 1) + 1);
           if (*aResult)
           {
diff --git a/netwerk/test/unit/test_MIME_params.js b/netwerk/test/unit/test_MIME_params.js
new file mode 100644
index 00000000000..f8bbdec0e7e
--- /dev/null
+++ b/netwerk/test/unit/test_MIME_params.js
@@ -0,0 +1,111 @@
+/**
+ * Test for bug 588781: ensure that for a MIME parameter with both "filename"
+ * and "filename*" we pick the RFC2231/5987 encoded form
+ *
+ * Note: RFC 5987 drops support for continuations (foo*0, foo*1*, etc.). 
+ *
+ * See also <http://greenbytes.de/tech/webdav/rfc5987.html#rfc.section.4.2>
+ */
+
+var succeed = [
+  ["Content-Disposition: attachment; filename=basic; filename*=UTF-8''extended",
+  "extended"],
+  ["Content-Disposition: attachment; filename*=UTF-8''extended; filename=basic",
+  "extended"],
+  ["Content-Disposition: attachment; filename=basic",
+  "basic"],
+  ["Content-Disposition: attachment; filename*=UTF-8''extended",
+  "extended"],
+  ["Content-Disposition: attachment; filename*0=foo; filename*1=bar",
+  "foobar"],
+/* BROKEN: we prepend 'basic' to result
+    ["Content-Disposition: attachment; filename=basic; filename*0=foo; filename*1=bar",
+    "foobar"],
+*/
+  // RFC 2231 not clear on correct outcome: we prefer non-continued extended
+  ["Content-Disposition: attachment; filename=basic; filename*=UTF-8''extended; filename*0=foo; filename*1=bar",
+  "extended"],
+/* BROKEN: not checking order yet 
+  // Gaps should result in returning only value until gap hit
+  ["Content-Disposition: attachment; filename*0=foo; filename*2=bar",
+  "foo"],
+*/
+/* BROKEN: don't check for leading 0s yet
+  // Don't handle leading 0's (*01)
+  ["Content-Disposition: attachment; filename*0=foo; filename*01=bar",
+  "foo"],
+*/
+  ["Content-Disposition: attachment; filename=basic; filename*0*=UTF-8''multi\r\n"
+    + " filename*1=line\r\n" 
+    + " filename*2*=%20extended",
+  "multiline extended"],
+/* BROKEN: not checking order yet 
+  // Gaps should result in returning only value until gap hit
+  ["Content-Disposition: attachment; filename=basic; filename*0*=UTF-8''multi\r\n"
+    + " filename*1=line\r\n" 
+    + " filename*3*=%20extended",
+  "multiline"],
+*/
+  // RFC 2231 not clear on correct outcome: we prefer non-continued extended
+  ["Content-Disposition: attachment; filename=basic; filename*0=UTF-8''multi\r\n"
+    + " filename*=UTF-8''extended\r\n"
+    + " filename*1=line\r\n" 
+    + " filename*2*=%20extended",
+  "extended"],
+  // sneaky: if unescaped, make sure we leave UTF-8'' in value
+  ["Content-Disposition: attachment; filename*0=UTF-8''unescaped\r\n"
+    + " filename*1*=%20so%20includes%20UTF-8''%20in%20value", 
+  "UTF-8''unescaped so includes UTF-8'' in value"],
+/* BROKEN: we prepend 'basic' to result
+  // sneaky: if unescaped, make sure we leave UTF-8'' in value
+  ["Content-Disposition: attachment; filename=basic; filename*0=UTF-8''unescaped\r\n"
+    + " filename*1*=%20so%20includes%20UTF-8''%20in%20value", 
+  "UTF-8''unescaped so includes UTF-8'' in value"],
+*/
+/*  BROKEN: we append filename*1 to 'basic'
+  // Also not sure if this is the spec'd behavior here:
+  ["Content-Disposition: attachment; filename=basic; filename*1=multi\r\n"
+    + " filename*2=line\r\n" 
+    + " filename*3*=%20extended",
+  "basic"],
+*/
+];
+
+var broken = [
+  ["Content-Disposition: attachment; filename*1=multi\r\n"
+    + " filename*2=line\r\n" 
+    + " filename*3*=%20extended",
+  "param continuation must start from 0: should fail"],
+];
+
+
+function run_test() {
+
+  var mhp = Components.classes["@mozilla.org/network/mime-hdrparam;1"]
+                      .getService(Components.interfaces.nsIMIMEHeaderParam);
+
+  var unused = { value : null };
+
+  for (var i = 0; i < succeed.length; ++i) {
+    dump("Testing " + succeed[i] + "\n");
+    try {
+      do_check_eq(mhp.getParameter(succeed[i][0], "filename", "UTF-8", true, unused),
+                  succeed[i][1]);
+    } catch (e) {}
+  }
+
+  // Check failure cases
+  for (var i = 0; i < broken.length; ++i) {
+    dump("Testing " + broken[i] + "\n");
+    try {
+      var result = mhp.getParameter(broken[i][0], "filename", "UTF-8", true, unused);
+      // No exception?  Error. 
+      do_check_eq(broken[i][1], "instead got: " + result);
+    } catch (e) {
+      // .result set if getParameter failed: check for correct error code
+      if (e.result)
+        do_check_eq(e.result, Components.results.NS_ERROR_OUT_OF_MEMORY);
+    }
+  }
+}
+