diff --git a/caps/NullPrincipalURI.cpp b/caps/NullPrincipalURI.cpp
index 739f4860cf5c..904e178eabe3 100644
--- a/caps/NullPrincipalURI.cpp
+++ b/caps/NullPrincipalURI.cpp
@@ -257,8 +257,8 @@ NullPrincipalURI::GetHasRef(bool* _result)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-NullPrincipalURI::SetSpec(const nsACString& aSpec)
+nsresult
+NullPrincipalURI::SetSpecInternal(const nsACString& aSpec)
 {
   return NS_ERROR_NOT_IMPLEMENTED;
 }
diff --git a/image/decoders/icon/nsIconURI.cpp b/image/decoders/icon/nsIconURI.cpp
index 5f096674bd35..69bfb70fe598 100644
--- a/image/decoders/icon/nsIconURI.cpp
+++ b/image/decoders/icon/nsIconURI.cpp
@@ -204,8 +204,8 @@ extractAttributeValue(const char* aSearchString,
   } // if we got non-null search string and attribute name values
 }
 
-NS_IMETHODIMP
-nsMozIconURI::SetSpec(const nsACString& aSpec)
+nsresult
+nsMozIconURI::SetSpecInternal(const nsACString& aSpec)
 {
   // Reset everything to default values.
   mIconURL = nullptr;
diff --git a/modules/libjar/nsJARURI.cpp b/modules/libjar/nsJARURI.cpp
index e546d2a93c98..0ea19afc2f4f 100644
--- a/modules/libjar/nsJARURI.cpp
+++ b/modules/libjar/nsJARURI.cpp
@@ -258,8 +258,8 @@ nsJARURI::GetHasRef(bool *result)
     return mJAREntry->GetHasRef(result);
 }
 
-NS_IMETHODIMP
-nsJARURI::SetSpec(const nsACString& aSpec)
+nsresult
+nsJARURI::SetSpecInternal(const nsACString& aSpec)
 {
     return SetSpecWithBase(aSpec, nullptr);
 }
diff --git a/netwerk/base/nsIURI.idl b/netwerk/base/nsIURI.idl
index a1a40c2495a5..8e9917194a5c 100644
--- a/netwerk/base/nsIURI.idl
+++ b/netwerk/base/nsIURI.idl
@@ -80,7 +80,16 @@ interface nsIURI : nsISupports
      */
 
     /**
-     * Returns a string representation of the URI. Setting the spec causes
+     * Returns a string representation of the URI.
+     *
+     * Some characters may be escaped.
+     */
+    readonly attribute AUTF8String spec;
+
+    /*
+     * This method should only be called by nsIURIMutator or C++ unit tests.
+     *
+     * Setting the spec causes
      * the new spec to be parsed per the rules for the scheme the URI
      * currently has.  In particular, setting the spec to a URI string with a
      * different scheme will generally produce incorrect results; no one
@@ -88,10 +97,9 @@ interface nsIURI : nsISupports
      * the URI stores information from the nsIIOService.newURI call used to
      * create it other than just the parsed string, then behavior of this
      * information on setting the spec attribute is undefined.
-     *
-     * Some characters may be escaped.
      */
-    attribute AUTF8String spec;
+    [noscript,notxpcom,nostdcall]
+    nsresult setSpecInternal(in ACString aSpec);
 
 %{ C++
     // An infallible wrapper for GetSpec() that returns a failure indication
diff --git a/netwerk/base/nsIURIMutator.idl b/netwerk/base/nsIURIMutator.idl
index d31384492bc9..687a691a83c1 100644
--- a/netwerk/base/nsIURIMutator.idl
+++ b/netwerk/base/nsIURIMutator.idl
@@ -75,7 +75,7 @@ protected:
       uri = new T();
     }
 
-    rv = uri->SetSpec(aSpec);
+    rv = uri->SetSpecInternal(aSpec);
     if (NS_FAILED(rv)) {
       return rv;
     }
diff --git a/netwerk/base/nsSimpleURI.cpp b/netwerk/base/nsSimpleURI.cpp
index d1e3c651db04..dd2802b75fe2 100644
--- a/netwerk/base/nsSimpleURI.cpp
+++ b/netwerk/base/nsSimpleURI.cpp
@@ -287,8 +287,8 @@ nsSimpleURI::GetHasRef(bool *result)
     return NS_OK;
 }
 
-NS_IMETHODIMP
-nsSimpleURI::SetSpec(const nsACString &aSpec)
+nsresult
+nsSimpleURI::SetSpecInternal(const nsACString &aSpec)
 {
     NS_ENSURE_STATE(mMutable);
 
diff --git a/netwerk/base/nsStandardURL.cpp b/netwerk/base/nsStandardURL.cpp
index b4ac62405a32..e11d312c8df6 100644
--- a/netwerk/base/nsStandardURL.cpp
+++ b/netwerk/base/nsStandardURL.cpp
@@ -1554,8 +1554,8 @@ IsSpecialProtocol(const nsACString &input)
            protocol.LowerCaseEqualsLiteral("gopher");
 }
 
-NS_IMETHODIMP
-nsStandardURL::SetSpec(const nsACString &input)
+nsresult
+nsStandardURL::SetSpecInternal(const nsACString &input)
 {
     return SetSpecWithEncoding(input, nullptr);
 }
@@ -2209,7 +2209,7 @@ nsStandardURL::SetPathQueryRef(const nsACString &input)
             spec.Append('/');
         spec.Append(path);
 
-        return SetSpec(spec);
+        return SetSpecInternal(spec);
     }
     else if (mPath.mLen >= 1) {
         mSpec.Cut(mPath.mPos + 1, mPath.mLen - 1);
@@ -2893,7 +2893,7 @@ nsStandardURL::SetFilePath(const nsACString &input)
                 spec.Append(mSpec.get() + end, mSpec.Length() - end);
         }
 
-        return SetSpec(spec);
+        return SetSpecInternal(spec);
     }
     else if (mPath.mLen > 1) {
         mSpec.Cut(mPath.mPos + 1, mFilepath.mLen - 1);