--- a/layout/style/nsCSSValue.cpp
+++ b/layout/style/nsCSSValue.cpp
@@ -2569,18 +2569,21 @@ css::URLValueData::operator==(const URLV
             eq)) &&
           (mOriginPrincipal == aOther.mOriginPrincipal ||
            self.mOriginPrincipal.get()->Equals(other.mOriginPrincipal.get()));
 }
 
 bool
 css::URLValueData::URIEquals(const URLValueData& aOther) const
 {
-  MOZ_ASSERT(mURIResolved && aOther.mURIResolved,
-             "How do you know the URIs aren't null?");
+  if (MOZ_UNLIKELY(!mURIResolved || !aOther.mURIResolved)) {
+    NS_WARNING("Comparing unresolved URIs");
+    return false;
+  }
+
   bool eq;
   // Cast away const so we can call nsIPrincipal::Equals.
   auto& self = *const_cast<URLValueData*>(this);
   auto& other = const_cast<URLValueData&>(aOther);
   // Worth comparing GetURI() to aOther.GetURI() and mOriginPrincipal to
   // aOther.mOriginPrincipal, because in the (probably common) case when this
   // value was one of the ones that in fact did not change this will be our
   // fast path to equality