diff --git a/Decimal.cpp b/Decimal.cpp
--- a/Decimal.cpp
+++ b/Decimal.cpp
@@ -28,12 +28,10 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include "platform/Decimal.h"
+#include "Decimal.h"
+#include "moz-decimal-utils.h"
 
-#include "wtf/Allocator.h"
-#include "wtf/MathExtras.h"
-#include "wtf/Noncopyable.h"
-#include "wtf/text/StringBuilder.h"
+using namespace moz_decimal_utils;
 
 #include <algorithm>
 #include <float.h>
@@ -695,7 +693,7 @@ Decimal Decimal::floor() const
 Decimal Decimal::fromDouble(double doubleValue)
 {
     if (std::isfinite(doubleValue))
-        return fromString(String::numberToStringECMAScript(doubleValue));
+        return fromString(mozToString(doubleValue));
 
     if (std::isinf(doubleValue))
         return infinity(doubleValue < 0 ? Negative : Positive);
@@ -936,7 +934,7 @@ double Decimal::toDouble() const
 {
     if (isFinite()) {
         bool valid;
-        const double doubleValue = toString().toDouble(&valid);
+        const double doubleValue = mozToDouble(toString(), &valid);
         return valid ? doubleValue : std::numeric_limits<double>::quiet_NaN();
     }
 
@@ -989,7 +987,7 @@ String Decimal::toString() const
         }
     }
 
-    const String digits = String::number(coefficient);
+    const String digits = mozToString(coefficient);
     int coefficientLength = static_cast<int>(digits.length());
     const int adjustedExponent = originalExponent + coefficientLength - 1;
     if (originalExponent <= 0 && adjustedExponent >= -6) {
diff --git a/Decimal.h b/Decimal.h
index f97da8c113e1..bc8264d1d5f5 100644
--- a/Decimal.h
+++ b/Decimal.h
@@ -28,16 +28,39 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+/**
+ * Imported from:
+ * https://chromium.googlesource.com/chromium/src.git/+/master/third_party/WebKit/Source/platform/Decimal.h
+ * Check UPSTREAM-GIT-SHA for the commit ID of the last update from Blink core.
+ */
+
 #ifndef Decimal_h
 #define Decimal_h
 
+#include "mozilla/Assertions.h"
+#include <stdint.h>
 #include "mozilla/Types.h"
 
-#include "platform/PlatformExport.h"
-#include "wtf/Allocator.h"
-#include "wtf/Assertions.h"
-#include "wtf/text/WTFString.h"
-#include <stdint.h>
+#include <string>
+
+#ifndef ASSERT
+#define DEFINED_ASSERT_FOR_DECIMAL_H 1
+#define ASSERT MOZ_ASSERT
+#endif
+
+#define PLATFORM_EXPORT
+
+// To use USING_FAST_MALLOC we'd need:
+// https://chromium.googlesource.com/chromium/src.git/+/master/third_party/WebKit/Source/wtf/Allocator.h
+// Since we don't allocate Decimal objects, no need.
+#define USING_FAST_MALLOC(type) \
+  void ignore_this_dummy_method() = delete
+
+#define DISALLOW_NEW()                                          \
+    private:                                                    \
+        void* operator new(size_t) = delete;                    \
+        void* operator new(size_t, void*) = delete;             \
+    public:
 
 namespace blink {
 
@@ -144,7 +167,7 @@ public:
 
     MFBT_API double toDouble() const;
     // Note: toString method supports infinity and nan but fromString not.
-    MFBT_API String toString() const;
+    MFBT_API std::string toString() const;
 
     static MFBT_API Decimal fromDouble(double);
     // fromString supports following syntax EBNF:
@@ -154,7 +177,7 @@ public:
     //  exponent-marker ::= 'e' | 'E'
     //  digit ::= '0' | '1' | ... | '9'
     // Note: fromString doesn't support "infinity" and "nan".
-    static MFBT_API Decimal fromString(const String&);
+    static MFBT_API Decimal fromString(const std::string& aValue);
     static MFBT_API Decimal infinity(Sign);
     static MFBT_API Decimal nan();
     static MFBT_API Decimal zero(Sign);
@@ -183,4 +206,15 @@ private:
 
 } // namespace blink
 
+namespace mozilla {
+typedef blink::Decimal Decimal;
+} // namespace mozilla
+
+#undef USING_FAST_MALLOC
+
+#ifdef DEFINED_ASSERT_FOR_DECIMAL_H
+#undef DEFINED_ASSERT_FOR_DECIMAL_H
+#undef ASSERT
+#endif
+
 #endif // Decimal_h