--- a/dom/canvas/WebGLContextValidate.cpp
+++ b/dom/canvas/WebGLContextValidate.cpp
@@ -490,19 +490,20 @@ WebGLContext::ValidateUniformArraySetter
         return false;
 
     if (!loc->ValidateSizeAndType(setterElemSize, setterType, this, funcName))
         return false;
 
     if (!loc->ValidateArrayLength(setterElemSize, setterArraySize, this, funcName))
         return false;
 
+    MOZ_ASSERT((size_t)loc->mActiveInfo->mElemCount > loc->mArrayIndex);
+    size_t uniformElemCount = loc->mActiveInfo->mElemCount - loc->mArrayIndex;
     *out_rawLoc = loc->mLoc;
-    *out_numElementsToUpload = std::min((size_t)loc->mActiveInfo->mElemCount,
-                                        setterArraySize / setterElemSize);
+    *out_numElementsToUpload = std::min(uniformElemCount, setterArraySize / setterElemSize);
     return true;
 }
 
 bool
 WebGLContext::ValidateUniformMatrixArraySetter(WebGLUniformLocation* loc,
                                                uint8_t setterCols,
                                                uint8_t setterRows,
                                                GLenum setterType,

--- a/dom/canvas/WebGLProgram.cpp
+++ b/dom/canvas/WebGLProgram.cpp
@@ -780,18 +780,24 @@ WebGLProgram::GetUniformLocation(const n
     if (!IsLinked()) {
         mContext->ErrorInvalidOperation("getUniformLocation: `program` must be linked.");
         return nullptr;
     }
 
     const NS_LossyConvertUTF16toASCII userName(userName_wide);
 
     nsDependentCString baseUserName;
-    bool isArray;
-    size_t arrayIndex;
+    bool isArray = false;
+    // GLES 2.0.25, Section 2.10, p35
+    // If the the uniform location is an array, then the location of the first
+    // element of that array can be retrieved by either using the name of the
+    // uniform array, or the name of the uniform array appended with "[0]".
+    // The ParseName() can't recognize this rule. So always initialize
+    // arrayIndex with 0.
+    size_t arrayIndex = 0;
     if (!ParseName(userName, &baseUserName, &isArray, &arrayIndex))
         return nullptr;
 
     const WebGLActiveInfo* activeInfo;
     if (!LinkInfo()->FindUniform(baseUserName, &activeInfo))
         return nullptr;
 
     const nsCString& baseMappedName = activeInfo->mBaseMappedName;
@@ -806,17 +812,18 @@ WebGLProgram::GetUniformLocation(const n
     gl::GLContext* gl = mContext->GL();
     gl->MakeCurrent();
 
     GLint loc = gl->fGetUniformLocation(mGLName, mappedName.BeginReading());
     if (loc == -1)
         return nullptr;
 
     RefPtr<WebGLUniformLocation> locObj = new WebGLUniformLocation(mContext, LinkInfo(),
-                                                                     loc, activeInfo);
+                                                                   loc, arrayIndex,
+                                                                   activeInfo);
     return locObj.forget();
 }
 
 void
 WebGLProgram::GetUniformIndices(const dom::Sequence<nsString>& uniformNames,
                                 dom::Nullable< nsTArray<GLuint> >& retval) const
 {
     size_t count = uniformNames.Length();

--- a/dom/canvas/WebGLUniformLocation.cpp
+++ b/dom/canvas/WebGLUniformLocation.cpp
@@ -11,20 +11,23 @@
 #include "WebGLActiveInfo.h"
 #include "WebGLContext.h"
 #include "WebGLProgram.h"
 
 namespace mozilla {
 
 WebGLUniformLocation::WebGLUniformLocation(WebGLContext* webgl,
                                            const webgl::LinkedProgramInfo* linkInfo,
-                                           GLuint loc, const WebGLActiveInfo* activeInfo)
+                                           GLuint loc,
+                                           size_t arrayIndex,
+                                           const WebGLActiveInfo* activeInfo)
     : WebGLContextBoundObject(webgl)
     , mLinkInfo(linkInfo)
     , mLoc(loc)
+    , mArrayIndex(arrayIndex)
     , mActiveInfo(activeInfo)
 { }
 
 WebGLUniformLocation::~WebGLUniformLocation()
 { }
 
 bool
 WebGLUniformLocation::ValidateForProgram(WebGLProgram* prog, WebGLContext* webgl,

--- a/dom/canvas/WebGLUniformLocation.h
+++ b/dom/canvas/WebGLUniformLocation.h
@@ -36,20 +36,21 @@ public:
     virtual JSObject* WrapObject(JSContext* js, JS::Handle<JSObject*> givenProto) override;
 
     WebGLContext* GetParentObject() const {
         return mContext;
     }
 
     const WeakPtr<const webgl::LinkedProgramInfo> mLinkInfo;
     const GLuint mLoc;
+    const size_t mArrayIndex;
     const WebGLActiveInfo* const mActiveInfo;
 
     WebGLUniformLocation(WebGLContext* webgl, const webgl::LinkedProgramInfo* linkInfo,
-                         GLuint loc, const WebGLActiveInfo* activeInfo);
+                         GLuint loc, size_t arrayIndex, const WebGLActiveInfo* activeInfo);
 
     bool ValidateForProgram(WebGLProgram* prog, WebGLContext* webgl,
                             const char* funcName) const;
     bool ValidateSamplerSetter(GLint value, WebGLContext* webgl,
                                const char* funcName) const;
     bool ValidateSizeAndType(uint8_t setterElemSize, GLenum setterType,
                              WebGLContext* webgl, const char* funcName) const;
     bool ValidateArrayLength(uint8_t setterElemSize, size_t setterArraySize,