diff --git a/config/static-checking-config.mk b/config/static-checking-config.mk
index 061f8daccbf3..ecdeb91a4eb0 100644
--- a/config/static-checking-config.mk
+++ b/config/static-checking-config.mk
@@ -5,6 +5,7 @@ DEHYDRA_SCRIPT = $(topsrcdir)/config/static-checking.js
 
 DEHYDRA_MODULES = \
   $(topsrcdir)/xpcom/analysis/final.js \
+  $(topsrcdir)/xpcom/analysis/override.js \
   $(topsrcdir)/xpcom/analysis/must-override.js \
   $(NULL)
 
diff --git a/config/static-checking.js b/config/static-checking.js
index f9961a3fce56..13ba4555fa11 100644
--- a/config/static-checking.js
+++ b/config/static-checking.js
@@ -62,6 +62,12 @@ function signaturesMatch(m1, m2)
 {
   if (m1.shortName != m2.shortName)
     return false;
+
+  if (m1.isVirtual != m2.isVirtual)
+    return false;
+  
+  if (m1.isStatic != m2.isStatic)
+    return false;
   
   let p1 = m1.type.parameters;
   let p2 = m2.type.parameters;
diff --git a/xpcom/analysis/override.js b/xpcom/analysis/override.js
new file mode 100644
index 000000000000..21201f946427
--- /dev/null
+++ b/xpcom/analysis/override.js
@@ -0,0 +1,42 @@
+/**
+ * NS_OVERRIDE may be marked on class methods which are intended to override
+ * a method in a base class. If the method is removed or altered in the base
+ * class, the compiler will force all subclass overrides to be modified.
+ */
+
+/**
+ * Generate all the base classes recursively of class `c`.
+ */
+function all_bases(c)
+{
+  for each (let b in c.bases) {
+    yield b.type;
+    for (let bb in all_bases(b.type))
+      yield bb;
+  }
+}
+
+function process_decl(d)
+{
+  if (!hasAttribute(d, 'NS_override'))
+    return;
+
+  if (!d.memberOf || !d.isFunction) {
+    error("%s is marked NS_OVERRIDE but is not a class function.".format(d.name), d.loc);
+    return;
+  }
+
+  if (d.isStatic) {
+    error("Marking NS_OVERRIDE on static function %s is meaningless.".format(d.name), d.loc);
+    return;
+  }
+
+  for (let base in all_bases(d.memberOf)) {
+    for each (let m in base.members) {
+      if (m.shortName == d.shortName && signaturesMatch(m, d))
+	return;
+    }
+  }
+
+  error("NS_OVERRIDE function %s does not override a base class method with the same name and signature".format(d.name), d.loc);
+}
diff --git a/xpcom/base/nscore.h b/xpcom/base/nscore.h
index c736658bbb82..7febe0b73de5 100644
--- a/xpcom/base/nscore.h
+++ b/xpcom/base/nscore.h
@@ -505,11 +505,13 @@ typedef PRUint32 nsrefcnt;
 # define NS_INPARAM __attribute__((user("NS_inparam")))
 # define NS_OUTPARAM  __attribute__((user("NS_outparam")))
 # define NS_INOUTPARAM __attribute__((user("NS_inoutparam")))
+# define NS_OVERRIDE __attribute__((user("NS_override")))
 #else
 # define NS_SCRIPTABLE
 # define NS_INPARAM
 # define NS_OUTPARAM
 # define NS_INOUTPARAM
+# define NS_OVERRIDE
 #endif
 
 #endif /* nscore_h___ */
diff --git a/xpcom/tests/static-checker/Makefile.in b/xpcom/tests/static-checker/Makefile.in
index 89ba23557208..05690b3bf1ea 100644
--- a/xpcom/tests/static-checker/Makefile.in
+++ b/xpcom/tests/static-checker/Makefile.in
@@ -124,10 +124,22 @@ MUST_OVERRIDE_FAILURE_TESTCASES = \
   OverrideFail4.cpp \
   $(NULL)
 
+OVERRIDE_PASS_TESTCASES = \
+  override-pass.cpp \
+  $(NULL)
+
+OVERRIDE_FAILURE_TESTCASES = \
+  override-global.cpp \
+  override-signature.cpp \
+  override-static.cpp \
+  override-virtual.cpp \
+  $(NULL)
+
 STATIC_FAILURE_TESTCASES = \
   $(FINAL_FAILURE_TESTCASES) \
   $(FLOW_FAILURE_TESTCASES) \
   $(MUST_OVERRIDE_FAILURE_TESTCASES) \
+  $(OVERRIDE_FAILURE_TESTCASES) \
   $(NULL)
 
 STATIC_WARNING_TESTCASES = \
@@ -141,6 +153,7 @@ STATIC_PASS_TESTCASES = \
   $(STACK_PASS_TESTCASES) \
   $(FLOW_PASS_TESTCASES) \
   $(MUST_OVERRIDE_PASS_TESTCASES) \
+  $(OVERRIDE_PASS_TESTCASES) \
   $(NULL)
 
 
diff --git a/xpcom/tests/static-checker/override-global.cpp b/xpcom/tests/static-checker/override-global.cpp
new file mode 100644
index 000000000000..e79d70c3f101
--- /dev/null
+++ b/xpcom/tests/static-checker/override-global.cpp
@@ -0,0 +1 @@
+__attribute__((user("NS_override"))) int m();
diff --git a/xpcom/tests/static-checker/override-pass.cpp b/xpcom/tests/static-checker/override-pass.cpp
new file mode 100644
index 000000000000..d5ac1e169897
--- /dev/null
+++ b/xpcom/tests/static-checker/override-pass.cpp
@@ -0,0 +1,15 @@
+class A
+{
+  int a(int, char*);
+  void c(char);
+};
+
+class B : A
+{
+  __attribute__((user("NS_override"))) int a(int, char*);
+};
+
+class C : B
+{
+  __attribute__((user("NS_override"))) void c(char);
+};
diff --git a/xpcom/tests/static-checker/override-signature.cpp b/xpcom/tests/static-checker/override-signature.cpp
new file mode 100644
index 000000000000..ec2c17aa63bc
--- /dev/null
+++ b/xpcom/tests/static-checker/override-signature.cpp
@@ -0,0 +1,9 @@
+class A
+{
+  int m(int);
+};
+
+class B : A
+{
+  __attribute__((user("NS_override"))) int m(void*);
+};
diff --git a/xpcom/tests/static-checker/override-static.cpp b/xpcom/tests/static-checker/override-static.cpp
new file mode 100644
index 000000000000..b56bd7fd3e6f
--- /dev/null
+++ b/xpcom/tests/static-checker/override-static.cpp
@@ -0,0 +1,9 @@
+class A
+{
+  static int m();
+};
+
+class B : A
+{
+  __attribute__((user("NS_override"))) static int m();
+};
diff --git a/xpcom/tests/static-checker/override-virtual.cpp b/xpcom/tests/static-checker/override-virtual.cpp
new file mode 100644
index 000000000000..020bfce47b97
--- /dev/null
+++ b/xpcom/tests/static-checker/override-virtual.cpp
@@ -0,0 +1,9 @@
+class A
+{
+  int m();
+};
+
+class B : A
+{
+  __attribute__((user("NS_override"))) virtual int m();
+};