diff --git a/netwerk/protocol/http/nsHttpHandler.cpp b/netwerk/protocol/http/nsHttpHandler.cpp
index a5c6fa511965..bf3d9a125f07 100644
--- a/netwerk/protocol/http/nsHttpHandler.cpp
+++ b/netwerk/protocol/http/nsHttpHandler.cpp
@@ -972,10 +972,12 @@ void nsHttpHandler::InitUserAgentComponents() {
   SInt32 majorVersion = nsCocoaFeatures::macOSVersionMajor();
   SInt32 minorVersion = nsCocoaFeatures::macOSVersionMinor();
 
+  // Cap the reported macOS version at 10.15 (like Safari) to avoid breaking
+  // sites that assume the UA's macOS version always begins with "10.".
+  int uaVersion = (majorVersion >= 11 || minorVersion > 15) ? 15 : minorVersion;
+
   // Always return an "Intel" UA string, even on ARM64 macOS like Safari does.
-  mOscpu =
-      nsPrintfCString("Intel Mac OS X %d.%d", static_cast<int>(majorVersion),
-                      static_cast<int>(minorVersion));
+  mOscpu = nsPrintfCString("Intel Mac OS X 10.%d", uaVersion);
 #  elif defined(XP_UNIX)
   struct utsname name;
   int ret = uname(&name);