--- a/toolkit/components/telemetry/Scalars.yaml
+++ b/toolkit/components/telemetry/Scalars.yaml
@@ -1763,16 +1763,33 @@ widget:
     expires: never
     kind: boolean
     notification_emails:
       - mnakano@mozilla.com
     release_channel_collection: opt-out
     record_in_processes:
       - 'main'
 
+  ime_name_on_linux:
+    bug_numbers:
+      - 1215818
+    description: >
+      Name of active IM (e.g., xim, fcitx, ibus, etc) which was actually set by
+      env on Linux.  Different from Windows and macOS, this value includes
+      non-IME users even though this is recoded when first compositionstart
+      event because dead key is also implemented by IME on Linux.
+    keyed: true
+    expires: never
+    kind: boolean
+    notification_emails:
+      - mnakano@mozilla.com
+    release_channel_collection: opt-out
+    record_in_processes:
+      - 'main'
+
 # The following section contains memory reporter counters.
 memoryreporter:
   max_ghost_windows:
     bug_numbers:
       - 1454724
     description: >
       The maximum number of leaked ghost windows seen.
     expires: "66"

--- a/widget/gtk/IMContextWrapper.cpp
+++ b/widget/gtk/IMContextWrapper.cpp
@@ -9,16 +9,17 @@
 
 #include "IMContextWrapper.h"
 #include "nsGtkKeyUtils.h"
 #include "nsWindow.h"
 #include "mozilla/AutoRestore.h"
 #include "mozilla/Likely.h"
 #include "mozilla/MiscEvents.h"
 #include "mozilla/Preferences.h"
+#include "mozilla/Telemetry.h"
 #include "mozilla/TextEventDispatcher.h"
 #include "mozilla/TextEvents.h"
 #include "WritingModes.h"
 
 namespace mozilla {
 namespace widget {
 
 LazyLogModule gGtkIMLog("nsGtkIMModuleWidgets");
@@ -309,16 +310,56 @@ static bool
 IsFcitxInSyncMode()
 {
     // See fcitx_im_context_class_init() in src/frontend/gtk2/fcitximcontext.c
     // https://github.com/fcitx/fcitx/blob/78b98d9230dc9630e99d52e3172bdf440ffd08c4/src/frontend/gtk2/fcitximcontext.c#L395-L398
     return GetFcitxBoolEnv("IBUS_ENABLE_SYNC_MODE") ||
            GetFcitxBoolEnv("FCITX_ENABLE_SYNC_MODE");
 }
 
+nsDependentCSubstring
+IMContextWrapper::GetIMName() const
+{
+    const char* contextIDChar =
+        gtk_im_multicontext_get_context_id(GTK_IM_MULTICONTEXT(mContext));
+    if (!contextIDChar) {
+        return nsDependentCSubstring();
+    }
+
+    nsDependentCSubstring im(contextIDChar, strlen(contextIDChar));
+
+    // If the context is XIM, actual engine must be specified with
+    // |XMODIFIERS=@im=foo|.
+    const char* xmodifiersChar = PR_GetEnv("XMODIFIERS");
+    if (!im.EqualsLiteral("xim") || !xmodifiersChar) {
+        return im;
+    }
+
+    nsDependentCString xmodifiers(xmodifiersChar);
+    int32_t atIMValueStart = xmodifiers.Find("@im=") + 4;
+    if (atIMValueStart < 4 ||
+        xmodifiers.Length() <= static_cast<size_t>(atIMValueStart)) {
+        return im;
+    }
+
+    int32_t atIMValueEnd =
+        xmodifiers.Find("@", false, atIMValueStart);
+    if (atIMValueEnd > atIMValueStart) {
+         return nsDependentCSubstring(xmodifiersChar + atIMValueStart,
+                                      atIMValueEnd - atIMValueStart);
+    }
+
+    if (atIMValueEnd == kNotFound) {
+        return nsDependentCSubstring(xmodifiersChar + atIMValueStart,
+                                     strlen(xmodifiersChar) - atIMValueStart);
+    }
+
+    return im;
+}
+
 void
 IMContextWrapper::Init()
 {
     MozContainer* container = mOwnerWindow->GetMozContainer();
     MOZ_ASSERT(container, "container is null");
     GdkWindow* gdkWindow = gtk_widget_get_window(GTK_WIDGET(container));
 
     // NOTE: gtk_im_*_new() abort (kill) the whole process when it fails.
@@ -334,41 +375,17 @@ IMContextWrapper::Init()
     g_signal_connect(mContext, "delete_surrounding",
         G_CALLBACK(IMContextWrapper::OnDeleteSurroundingCallback), this);
     g_signal_connect(mContext, "commit",
         G_CALLBACK(IMContextWrapper::OnCommitCompositionCallback), this);
     g_signal_connect(mContext, "preedit_start",
         G_CALLBACK(IMContextWrapper::OnStartCompositionCallback), this);
     g_signal_connect(mContext, "preedit_end",
         G_CALLBACK(IMContextWrapper::OnEndCompositionCallback), this);
-    nsDependentCSubstring im;
-    const char* contextIDChar =
-        gtk_im_multicontext_get_context_id(GTK_IM_MULTICONTEXT(mContext));
-    const char* xmodifiersChar = PR_GetEnv("XMODIFIERS");
-    if (contextIDChar) {
-        im.Rebind(contextIDChar, strlen(contextIDChar));
-        // If the context is XIM, actual engine must be specified with
-        // |XMODIFIERS=@im=foo|.
-        if (im.EqualsLiteral("xim") && xmodifiersChar) {
-            nsDependentCString xmodifiers(xmodifiersChar);
-            int32_t atIMValueStart = xmodifiers.Find("@im=") + 4;
-            if (atIMValueStart >= 4 &&
-                xmodifiers.Length() > static_cast<size_t>(atIMValueStart)) {
-                int32_t atIMValueEnd =
-                    xmodifiers.Find("@", false, atIMValueStart);
-                if (atIMValueEnd > atIMValueStart) {
-                    im.Rebind(xmodifiersChar + atIMValueStart,
-                              atIMValueEnd - atIMValueStart);
-                } else if (atIMValueEnd == kNotFound) {
-                    im.Rebind(xmodifiersChar + atIMValueStart,
-                              strlen(xmodifiersChar) - atIMValueStart);
-                }
-            }
-        }
-    }
+    nsDependentCSubstring im = GetIMName();
     if (im.EqualsLiteral("ibus")) {
         mIMContextID = IMContextID::eIBus;
         mIsIMInAsyncKeyHandlingMode = !IsIBusInSyncMode();
         // Although ibus has key snooper mode, it's forcibly disabled on Firefox
         // in default settings by its whitelist since we always send key events
         // to IME before handling shortcut keys.  The whitelist can be
         // customized with env, IBUS_NO_SNOOPER_APPS, but we don't need to
         // support such rare cases for reducing maintenance cost.
@@ -431,21 +448,24 @@ IMContextWrapper::Init()
 
     // Dummy context
     mDummyContext = gtk_im_multicontext_new();
     gtk_im_context_set_client_window(mDummyContext, gdkWindow);
 
     MOZ_LOG(gGtkIMLog, LogLevel::Info,
         ("0x%p Init(), mOwnerWindow=%p, mContext=%p (im=\"%s\"), "
          "mIsIMInAsyncKeyHandlingMode=%s, mIsKeySnooped=%s, "
-         "mSimpleContext=%p, mDummyContext=%p, contextIDChar=\"%s\", "
-         "xmodifiersChar=\"%s\"",
+         "mSimpleContext=%p, mDummyContext=%p, "
+         "gtk_im_multicontext_get_context_id()=\"%s\", "
+         "PR_GetEnv(\"XMODIFIERS\")=\"%s\"",
          this, mOwnerWindow, mContext, nsAutoCString(im).get(),
          ToChar(mIsIMInAsyncKeyHandlingMode), ToChar(mIsKeySnooped),
-         mSimpleContext, mDummyContext, contextIDChar, xmodifiersChar));
+         mSimpleContext, mDummyContext,
+         gtk_im_multicontext_get_context_id(GTK_IM_MULTICONTEXT(mContext)),
+         PR_GetEnv("XMODIFIERS")));
 }
 
 IMContextWrapper::~IMContextWrapper()
 {
     if (this == sLastFocusedContext) {
         sLastFocusedContext = nullptr;
     }
     MOZ_LOG(gGtkIMLog, LogLevel::Info,
@@ -1971,16 +1991,35 @@ IMContextWrapper::DispatchCompositionSta
     if (NS_WARN_IF(NS_FAILED(rv))) {
         MOZ_LOG(gGtkIMLog, LogLevel::Error,
             ("0x%p   DispatchCompositionStart(), FAILED, "
              "due to BeginNativeInputTransaction() failure",
              this));
         return false;
     }
 
+    static bool sHasSetTelemetry = false;
+    if (!sHasSetTelemetry) {
+        sHasSetTelemetry = true;
+        NS_ConvertUTF8toUTF16 im(GetIMName());
+        // 72 is kMaximumKeyStringLength in TelemetryScalar.cpp
+        if (im.Length() > 72) {
+            if (NS_IS_LOW_SURROGATE(im[72 - 1]) &&
+                NS_IS_HIGH_SURROGATE(im[72 - 2])) {
+                im.Truncate(72 - 2);
+            } else {
+                im.Truncate(72 - 1);
+            }
+            // U+2026 is "..."
+            im.Append(char16_t(0x2026));
+        }
+        Telemetry::ScalarSet(Telemetry::ScalarID::WIDGET_IME_NAME_ON_LINUX,
+                             im, true);
+    }
+
     MOZ_LOG(gGtkIMLog, LogLevel::Debug,
         ("0x%p   DispatchCompositionStart(), dispatching "
          "compositionstart... (mCompositionStart=%u)",
          this, mCompositionStart));
     mCompositionState = eCompositionState_CompositionStartDispatched;
     nsEventStatus status;
     dispatcher->StartComposition(status);
     if (lastFocusedWindow->IsDestroyed() ||

--- a/widget/gtk/IMContextWrapper.h
+++ b/widget/gtk/IMContextWrapper.h
@@ -123,16 +123,22 @@ public:
                 return "eScim";
             case IMContextID::eUim:
                 return "eUim";
             default:
                 return "eUnknown";
         }
     }
 
+    /**
+     * GetIMName() returns IM name associated with mContext.  If the context is
+     * xim, this look for actual engine from XMODIFIERS environment variable.
+     */
+    nsDependentCSubstring GetIMName() const;
+
 protected:
     ~IMContextWrapper();
 
     // Owner of an instance of this class. This should be top level window.
     // The owner window must release the contexts when it's destroyed because
     // the IME contexts need the native window.  If OnDestroyWindow() is called
     // with the owner window, it'll release IME contexts.  Otherwise, it'll
     // just clean up any existing composition if it's related to the destroying