diff --git a/components/l10n/messagebundles.cpp b/components/l10n/messagebundles.cpp
index 7d37b4358f..7a0799f21d 100644
--- a/components/l10n/messagebundles.cpp
+++ b/components/l10n/messagebundles.cpp
@@ -2,6 +2,7 @@
 
 #include <charconv>
 #include <cstring>
+#include <mutex>
 #include <optional>
 #include <span>
 
@@ -257,17 +258,24 @@ namespace L10n
 
     const icu::MessageFormat* MessageBundles::findMessage(std::string_view key, std::string_view localeName) const
     {
-        auto iter = mBundles.find(localeName);
-        if (iter != mBundles.end())
+        std::shared_lock sharedLock(mMutex);
         {
-            auto message = iter->second.find(key);
-            if (message != iter->second.end())
+            auto iter = mBundles.find(localeName);
+            if (iter != mBundles.end())
             {
-                return &(message->second);
+                auto message = iter->second.find(key);
+                if (message != iter->second.end())
+                {
+                    return &(message->second);
+                }
             }
         }
         if (localeName == "gmst" && mGmstLoader)
         {
+            if (!mGmsts.contains(key))
+                return nullptr;
+            sharedLock.unlock();
+            std::unique_lock lock(mMutex);
             auto found = mGmsts.find(key);
             if (found != mGmsts.end())
             {
@@ -275,6 +283,7 @@ namespace L10n
                 mGmsts.erase(found);
                 if (message)
                 {
+                    auto iter = mBundles.find(localeName);
                     if (iter == mBundles.end())
                         iter = mBundles.emplace(localeName, StringMap<icu::MessageFormat>()).first;
                     return &iter->second.emplace(key, *message).first->second;
diff --git a/components/l10n/messagebundles.hpp b/components/l10n/messagebundles.hpp
index ef4ce49cee..ac67c94ff0 100644
--- a/components/l10n/messagebundles.hpp
+++ b/components/l10n/messagebundles.hpp
@@ -3,6 +3,7 @@
 
 #include <functional>
 #include <map>
+#include <shared_mutex>
 #include <string_view>
 #include <unordered_map>
 #include <vector>
@@ -65,6 +66,7 @@ namespace L10n
         // icu::Locale isn't hashable (or comparable), so we use the string form instead, which is canonicalized
         mutable StringMap<StringMap<icu::MessageFormat>> mBundles;
         mutable StringMap<GmstMessageFormat> mGmsts;
+        mutable std::shared_mutex mMutex;
         const icu::Locale mFallbackLocale;
         std::vector<std::string> mPreferredLocaleStrings;
         std::vector<icu::Locale> mPreferredLocales;