diff --git a/dom/canvas/WebGLElementArrayCache.cpp b/dom/canvas/WebGLElementArrayCache.cpp
index 0c2502809a3..41a198c2ca6 100644
--- a/dom/canvas/WebGLElementArrayCache.cpp
+++ b/dom/canvas/WebGLElementArrayCache.cpp
@@ -19,7 +19,10 @@ static void
 UpdateUpperBound(uint32_t* const out_upperBound, uint32_t newBound)
 {
     MOZ_ASSERT(out_upperBound);
-    *out_upperBound = std::max(*out_upperBound, newBound);
+    // Move *out_upperBound to a local variable to work around a false positive
+    // -Wuninitialized gcc warning about std::max() in PGO builds.
+    uint32_t upperBound = *out_upperBound;
+    *out_upperBound = std::max(upperBound, newBound);
 }
 
 /* WebGLElementArrayCacheTree contains most of the implementation of
diff --git a/dom/canvas/compiledtest/TestWebGLElementArrayCache.cpp b/dom/canvas/compiledtest/TestWebGLElementArrayCache.cpp
index 242cf4f2c1e..33567289808 100644
--- a/dom/canvas/compiledtest/TestWebGLElementArrayCache.cpp
+++ b/dom/canvas/compiledtest/TestWebGLElementArrayCache.cpp
@@ -115,7 +115,7 @@ CheckSanity()
                         // ensure we exercise some nontrivial tree-walking
   T data[numElems] = {1,0,3,1,2,6,5,4}; // intentionally specify only 8 elements for now
   size_t numBytes = numElems * sizeof(T);
-  MOZ_ASSERT(numBytes == sizeof(data));
+  MOZ_RELEASE_ASSERT(numBytes == sizeof(data));
 
   GLenum type = GLType<T>();
 
@@ -139,7 +139,7 @@ CheckSanity()
   CheckValidate(true,  c, type, numElems,     0, numElems);
   CheckValidate(false, c, type, numElems - 1, 0, numElems);
 
-  MOZ_ASSERT(numElems > 10);
+  MOZ_RELEASE_ASSERT(numElems > 10);
   CheckValidate(true,  c, type, numElems - 10, 10, numElems - 10);
   CheckValidate(false, c, type, numElems - 11, 10, numElems - 10);
 }
@@ -156,7 +156,7 @@ CheckUintOverflow()
                               // ensure we exercise some nontrivial tree-walking
   T data[numElems];
   size_t numBytes = numElems * sizeof(T);
-  MOZ_ASSERT(numBytes == sizeof(data));
+  MOZ_RELEASE_ASSERT(numBytes == sizeof(data));
 
   GLenum type = GLType<T>();