--- a/ipc/chromium/src/chrome/common/ipc_message.h
+++ b/ipc/chromium/src/chrome/common/ipc_message.h
@@ -60,16 +60,17 @@ class Message : public Pickle {
     NESTED_INSIDE_SYNC = 2,
     NESTED_INSIDE_CPOW = 3
   };
 
   enum PriorityValue {
     NORMAL_PRIORITY = 0,
     INPUT_PRIORITY = 1,
     HIGH_PRIORITY = 2,
+    MEDIUMHIGH_PRIORITY = 3,
   };
 
   enum MessageCompression {
     COMPRESSION_NONE,
     COMPRESSION_ENABLED,
     COMPRESSION_ALL
   };
 

--- a/ipc/glue/MessageChannel.cpp
+++ b/ipc/glue/MessageChannel.cpp
@@ -2067,16 +2067,19 @@ MessageChannel::MessageTask::GetPriority
       *aPriority = PRIORITY_NORMAL;
       break;
     case Message::INPUT_PRIORITY:
       *aPriority = PRIORITY_INPUT;
       break;
     case Message::HIGH_PRIORITY:
       *aPriority = PRIORITY_HIGH;
       break;
+    case Message::MEDIUMHIGH_PRIORITY:
+      *aPriority = PRIORITY_MEDIUMHIGH;
+      break;
     default:
       MOZ_ASSERT(false);
       break;
   }
   return NS_OK;
 }
 
 NS_IMETHODIMP

--- a/ipc/ipdl/ipdl/ast.py
+++ b/ipc/ipdl/ipdl/ast.py
@@ -4,17 +4,17 @@
 
 NOT_NESTED = 1
 INSIDE_SYNC_NESTED = 2
 INSIDE_CPOW_NESTED = 3
 
 NORMAL_PRIORITY = 1
 INPUT_PRIORITY = 2
 HIGH_PRIORITY = 3
-
+MEDIUMHIGH_PRIORITY = 4
 
 class Visitor:
     def defaultVisit(self, node):
         raise Exception("INTERNAL ERROR: no visitor for node type `%s'" %
                         (node.__class__.__name__))
 
     def visitTranslationUnit(self, tu):
         for cxxInc in tu.cxxIncludes:

--- a/ipc/ipdl/ipdl/lower.py
+++ b/ipc/ipdl/ipdl/lower.py
@@ -1794,18 +1794,20 @@ def _generateMessageConstructor(md, segm
     else:
         assert nested == ipdl.ast.INSIDE_CPOW_NESTED
         nestedEnum = 'NESTED_INSIDE_CPOW'
 
     if prio == ipdl.ast.NORMAL_PRIORITY:
         prioEnum = 'NORMAL_PRIORITY'
     elif prio == ipdl.ast.INPUT_PRIORITY:
         prioEnum = 'INPUT_PRIORITY'
+    elif prio == ipdl.ast.HIGH_PRIORITY:
+        prioEnum = 'HIGH_PRIORITY'
     else:
-        prioEnum = 'HIGH_PRIORITY'
+        prioEnum = 'MEDIUMHIGH_PRIORITY'
 
     if md.decl.type.isSync():
         syncEnum = 'SYNC'
     else:
         syncEnum = 'ASYNC'
 
     if md.decl.type.isInterrupt():
         interruptEnum = 'INTERRUPT'

--- a/ipc/ipdl/ipdl/parser.py
+++ b/ipc/ipdl/ipdl/parser.py
@@ -558,19 +558,20 @@ def p_Nested(p):
 
     p[0] = {'nested': kinds[p[1]]}
 
 
 def p_Priority(p):
     """Priority : ID"""
     kinds = {'normal': 1,
              'input': 2,
-             'high': 3}
+             'high': 3,
+             'mediumhigh': 4}
     if p[1] not in kinds:
-        _error(locFromTok(p, 1), "Expected normal or high for prio()")
+        _error(locFromTok(p, 1), "Expected normal, input, high or mediumhigh for prio()")
 
     p[0] = {'prio': kinds[p[1]]}
 
 
 def p_SendQualifier(p):
     """SendQualifier : NESTED '(' Nested ')'
                      | PRIO '(' Priority ')'"""
     p[0] = p[3]

--- a/ipc/ipdl/sync-messages.ini
+++ b/ipc/ipdl/sync-messages.ini
@@ -136,16 +136,18 @@ description =
 [PTestLatency::Synchro2]
 description =
 [PTestNestedLoops::R]
 description =
 [PTestPriority::PMsg2]
 description =
 [PTestPriority::PMsg4]
 description =
+[PTestPriority::PMsg6]
+description =
 [PTestRPC::Test1_Start]
 description =
 [PTestRPC::Test1_InnerEvent]
 description =
 [PTestRPC::Test2_OutOfOrder]
 description =
 [PTestRPC::Test1_InnerQuery]
 description =

--- a/ipc/ipdl/test/cxx/PTestPriority.ipdl
+++ b/ipc/ipdl/test/cxx/PTestPriority.ipdl
@@ -2,16 +2,19 @@ namespace mozilla {
 namespace _ipdltest {
 
 sync protocol PTestPriority {
 parent:
     prio(input) async PMsg1();
     prio(input) sync PMsg2();
     prio(high) async PMsg3();
     prio(high) sync PMsg4();
+    prio(mediumhigh) async PMsg5();
+    prio(mediumhigh) sync PMsg6();
 
 child:
     prio(input) async CMsg1();
     prio(high) async CMsg2();
+    prio(mediumhigh) async CMsg3();
 };
 
 } // namespace _ipdltest
 } // namespace mozilla

--- a/ipc/ipdl/test/ipdl/ok/Prio.ipdl
+++ b/ipc/ipdl/test/ipdl/ok/Prio.ipdl
@@ -1,7 +1,8 @@
 async protocol Prio
 {
 child:
   prio(normal) async NormalPrio();
   prio(high) async HighPrio();
   prio(input) async InputPrio();
+  prio(mediumhigh) async MediumHighPrio();
 };