--- a/gfx/thebes/gfxEnv.h
+++ b/gfx/thebes/gfxEnv.h
@@ -86,17 +86,17 @@ public:
   DECL_GFX_ENV_LIVE("MOZ_GL_DEBUG_ABORT_ON_ERROR", GlDebugAbortOnError);
 
   // Count GL extensions
   DECL_GFX_ENV_ONCE("MOZ_GL_DUMP_EXTS", GlDumpExtensions);
 
   // Very noisy GLContext and GLContextProviderELG
   DECL_GFX_ENV_ONCE("MOZ_GL_SPEW", GlSpew);
 
-  // Do extra work before and after each GLX call in GLContextProviderGLX 
+  // Do extra work before and after each GLX call in GLContextProviderGLX
   DECL_GFX_ENV_LIVE("MOZ_GLX_DEBUG", GlxDebug);
 
   // Use X compositing
   DECL_GFX_ENV_LIVE("MOZ_LAYERS_ENABLE_XLIB_SURFACES", LayersEnableXlibSurfaces);
 
   // GL compositing on Windows
   DECL_GFX_ENV_LIVE("MOZ_LAYERS_PREFER_EGL", LayersPreferEGL);
 
@@ -108,17 +108,17 @@ public:
 
   // WARNING:
   // Please make sure that you've added your new envvar to the list above in
   // alphabetical order. Please do not just append it to the end of the list.
 
 private:
   // Helper function, can be re-used in the other macros
   static bool IsEnvSet(const char* aName) {
-    char* val = PR_GetEnv(aName);
+    const char* val = PR_GetEnv(aName);
     return (val != 0 && *val != '\0');
   }
 
   gfxEnv() {};
   ~gfxEnv() {};
   gfxEnv(const gfxEnv&) = delete;
   gfxEnv& operator=(const gfxEnv&) = delete;
 };