diff -up firefox-124.0.1/widget/gtk/IMContextWrapper.cpp.im-patch firefox-124.0.1/widget/gtk/IMContextWrapper.cpp
--- firefox-124.0.1/widget/gtk/IMContextWrapper.cpp.im-patch	2024-03-22 05:00:19.000000000 +0100
+++ firefox-124.0.1/widget/gtk/IMContextWrapper.cpp	2024-03-25 21:39:58.961947576 +0100
@@ -232,11 +232,18 @@ class SelectionStyleProvider final {
     sHasShutDown = true;
   }
 
-  // mContainer associated with an IM context.
-  void AttachTo(MozContainer* aContainer) {
-    gtk_style_context_add_provider(
-        gtk_widget_get_style_context(GTK_WIDGET(aContainer)),
-        GTK_STYLE_PROVIDER(mProvider), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+  // aGDKWindow is a GTK window which will be associated with an IM context.
+  void AttachTo(GdkWindow* aGDKWindow) {
+    GtkWidget* widget = nullptr;
+    // gdk_window_get_user_data() typically returns pointer to widget that
+    // window belongs to.  If it's widget, fcitx retrieves selection colors
+    // of them.  So, we need to overwrite its style.
+    gdk_window_get_user_data(aGDKWindow, (gpointer*)&widget);
+    if (GTK_IS_WIDGET(widget)) {
+      gtk_style_context_add_provider(gtk_widget_get_style_context(widget),
+                                     GTK_STYLE_PROVIDER(mProvider),
+                                     GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+    }
   }
 
   void OnThemeChanged() {
@@ -410,17 +417,21 @@ nsDependentCSubstring IMContextWrapper::
 }
 
 void IMContextWrapper::Init() {
+  MozContainer* container = mOwnerWindow->GetMozContainer();
+  MOZ_ASSERT(container, "container is null");
+  GdkWindow* gdkWindow = gtk_widget_get_window(GTK_WIDGET(container));
+
   // Overwrite selection colors of the window before associating the window
   // with IM context since IME may look up selection colors via IM context
   // to support any colored widgets.
-  SelectionStyleProvider::GetInstance()->AttachTo(
-      mOwnerWindow->GetMozContainer());
+  SelectionStyleProvider::GetInstance()->AttachTo(gdkWindow);
 
   // NOTE: gtk_im_*_new() abort (kill) the whole process when it fails.
   //       So, we don't need to check the result.
 
   // Normal context.
   mContext = gtk_im_multicontext_new();
+  gtk_im_context_set_client_window(mContext, gdkWindow);
   g_signal_connect(mContext, "preedit_changed",
                    G_CALLBACK(IMContextWrapper::OnChangeCompositionCallback),
                    this);
@@ -492,6 +503,7 @@ void IMContextWrapper::Init() {
   // Simple context
   if (sUseSimpleContext) {
     mSimpleContext = gtk_im_context_simple_new();
+    gtk_im_context_set_client_window(mSimpleContext, gdkWindow);
     g_signal_connect(mSimpleContext, "preedit_changed",
                      G_CALLBACK(&IMContextWrapper::OnChangeCompositionCallback),
                      this);
@@ -514,6 +526,7 @@ void IMContextWrapper::Init() {
 
   // Dummy context
   mDummyContext = gtk_im_multicontext_new();
+  gtk_im_context_set_client_window(mDummyContext, gdkWindow);
 
   MOZ_LOG(gIMELog, LogLevel::Info,
           ("0x%p Init(), mOwnerWindow=%p, mContext=%p (im=\"%s\"), "
@@ -540,17 +553,6 @@ IMContextWrapper::~IMContextWrapper() {
   MOZ_LOG(gIMELog, LogLevel::Info, ("0x%p ~IMContextWrapper()", this));
 }
 
-void IMContextWrapper::SetGdkWindow(GdkWindow* aGdkWindow) {
-  MOZ_LOG(gIMELog, LogLevel::Info,
-          ("0x%p GdkWindowChanged(%p)", this, aGdkWindow));
-  MOZ_ASSERT(!aGdkWindow || mOwnerWindow->GetGdkWindow() == aGdkWindow);
-  gtk_im_context_set_client_window(mContext, aGdkWindow);
-  if (mSimpleContext) {
-    gtk_im_context_set_client_window(mSimpleContext, aGdkWindow);
-  }
-  gtk_im_context_set_client_window(mDummyContext, aGdkWindow);
-}
-
 NS_IMETHODIMP
 IMContextWrapper::NotifyIME(TextEventDispatcher* aTextEventDispatcher,
                             const IMENotification& aNotification) {
diff -up firefox-124.0.1/widget/gtk/IMContextWrapper.h.im-patch firefox-124.0.1/widget/gtk/IMContextWrapper.h
--- firefox-124.0.1/widget/gtk/IMContextWrapper.h.im-patch	2024-03-22 05:00:19.000000000 +0100
+++ firefox-124.0.1/widget/gtk/IMContextWrapper.h	2024-03-25 21:39:58.961947576 +0100
@@ -117,10 +117,6 @@ class IMContextWrapper final : public Te
   void OnUpdateComposition();
   void OnLayoutChange();
 
-  // Set GdkWindow associated with IM context.
-  // It can be null which disables context operations.
-  void SetGdkWindow(GdkWindow* aGdkWindow);
-
   TextEventDispatcher* GetTextEventDispatcher();
 
   // TODO: Typically, new IM comes every several years.  And now, our code
diff -up firefox-124.0.1/widget/gtk/nsWindow.cpp.im-patch firefox-124.0.1/widget/gtk/nsWindow.cpp
--- firefox-124.0.1/widget/gtk/nsWindow.cpp.im-patch	2024-03-25 21:39:58.959947505 +0100
+++ firefox-124.0.1/widget/gtk/nsWindow.cpp	2024-03-25 21:41:09.178439036 +0100
@@ -5822,9 +5822,6 @@ void nsWindow::EnsureGdkWindow() {
   if (!mGdkWindow) {
     mGdkWindow = gtk_widget_get_window(GTK_WIDGET(mContainer));
     g_object_set_data(G_OBJECT(mGdkWindow), "nsWindow", this);
-    if (mIMContext) {
-      mIMContext->SetGdkWindow(mGdkWindow);
-    }
   }
 }
 
@@ -9933,9 +9930,6 @@ void nsWindow::DisableRendering() {
   LOG("nsWindow::DisableRendering()");
 
   if (mGdkWindow) {
-    if (mIMContext) {
-      mIMContext->SetGdkWindow(nullptr);
-    }
     g_object_set_data(G_OBJECT(mGdkWindow), "nsWindow", nullptr);
     mGdkWindow = nullptr;
   }