--- a/dom/events/TextComposition.cpp
+++ b/dom/events/TextComposition.cpp
@@ -262,20 +262,23 @@ TextComposition::DispatchCompositionEven
                                 aCompositionEvent->mRanges);
   }
   if (aCompositionEvent->mMessage == eCompositionCommitAsIs) {
     NS_ASSERTION(!aCompositionEvent->mRanges,
                  "mRanges of eCompositionCommitAsIs should be null");
     aCompositionEvent->mRanges = nullptr;
     NS_ASSERTION(aCompositionEvent->mData.IsEmpty(),
                  "mData of eCompositionCommitAsIs should be empty string");
-    if (mLastData == IDEOGRAPHIC_SPACE) {
-      // If the last data is an ideographic space (FullWidth space), it must be
+    bool removePlaceholderCharacter =
+      Preferences::GetBool("intl.ime.remove_placeholder_character_at_commit",
+                           false);
+    if (removePlaceholderCharacter && mLastData == IDEOGRAPHIC_SPACE) {
+      // If the last data is an ideographic space (FullWidth space), it might be
       // a placeholder character of some Chinese IME.  So, committing with
-      // this data must not be expected by users.  Let's use empty string.
+      // this data might not be expected by users.  Let's use empty string.
       aCompositionEvent->mData.Truncate();
     } else {
       aCompositionEvent->mData = mLastData;
     }
   } else if (aCompositionEvent->mMessage == eCompositionCommit) {
     NS_ASSERTION(!aCompositionEvent->mRanges,
                  "mRanges of eCompositionCommit should be null");
     aCompositionEvent->mRanges = nullptr;

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1972,16 +1972,22 @@ pref("font.language.group",             
 
 // Android-specific pref to use key-events-only mode for IME-unaware webapps.
 #ifdef MOZ_WIDGET_ANDROID
 pref("intl.ime.hack.on_ime_unaware_apps.fire_key_events_for_composition", true);
 #else
 pref("intl.ime.hack.on_ime_unaware_apps.fire_key_events_for_composition", false);
 #endif
 
+// If you use legacy Chinese IME which puts an ideographic space to composition
+// string as placeholder, this pref might be useful.  If this is true and when
+// web contents forcibly commits composition (e.g., moving focus), the
+// ideographic space will be ignored (i.e., commits with empty string).
+pref("intl.ime.remove_placeholder_character_at_commit", false);
+
 // these locales have right-to-left UI
 pref("intl.uidirection.ar", "rtl");
 pref("intl.uidirection.he", "rtl");
 pref("intl.uidirection.fa", "rtl");
 pref("intl.uidirection.ug", "rtl");
 pref("intl.uidirection.ur", "rtl");
 
 // use en-US hyphenation by default for content tagged with plain lang="en"