--- a/xpcom/base/nsSystemInfo.cpp
+++ b/xpcom/base/nsSystemInfo.cpp
@@ -335,16 +335,20 @@ GetProcessorInformation(int* physical_cp
   }
   return;
 }
 #endif
 
 nsresult
 nsSystemInfo::Init()
 {
+  // This uses the observer service on Windows, so for simplicity
+  // check that it is called from the main thread on all platforms.
+  MOZ_ASSERT(NS_IsMainThread());
+
   nsresult rv;
 
   static const struct
   {
     PRSysInfo cmd;
     const char* name;
   } items[] = {
     { PR_SI_SYSNAME, "name" },