--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -6833,124 +6833,16 @@
     "description": "Labeling status of top-level page when overriding charset (0: unlabeled file URL without detection, 1: unlabeled non-TLD-guessed non-file URL without detection, 2: unlabeled file URL with detection, 3: unlabeled non-file URL with detection, 4: labeled, 5: already overridden, 6: bug, 7: unlabeled with TLD guessing)"
   },
   "CHARSET_OVERRIDE_USED": {
     "record_in_processes": ["main", "content"],
     "expires_in_version": "never",
     "kind": "flag",
     "description": "Whether the character encoding menu was used to override an encoding in this session."
   },
-  "DECODER_INSTANTIATED_ISO2022JP": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for ISO-2022-JP has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_IBM866": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for IBM866 has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACGREEK": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACGREEK has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACICELANDIC": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACICELANDIC has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACCE": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACCE has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACHEBREW": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACHEBREW has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACARABIC": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACARABIC has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACFARSI": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACFARSI has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACCROATIAN": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACCROATIAN has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACCYRILLIC": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACCYRILLIC has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACROMANIAN": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACROMANIAN has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACTURKISH": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACTURKISH has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACDEVANAGARI": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACDEVANAGARI has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACGUJARATI": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACGUJARATI has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_MACGURMUKHI": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for MACGURMUKHI has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_KOI8R": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for KOI8R has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_KOI8U": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for KOI8U has been instantiated in this session."
-  },
-  "DECODER_INSTANTIATED_ISO_8859_5": {
-    "record_in_processes": ["main", "content"],
-    "expires_in_version": "never",
-    "kind": "flag",
-    "description": "Whether the decoder for ISO-8859-5 has been instantiated in this session."
-  },
   "LONG_REFLOW_INTERRUPTIBLE": {
     "record_in_processes": ["main", "content"],
     "expires_in_version": "never",
     "kind": "boolean",
     "description": "Long running reflow, interruptible or not"
   },
   "XMLHTTPREQUEST_ASYNC_OR_SYNC": {
     "record_in_processes": ["main", "content"],

--- a/toolkit/components/telemetry/histogram-whitelists.json
+++ b/toolkit/components/telemetry/histogram-whitelists.json
@@ -99,34 +99,16 @@
     "COMPONENTS_SHIM_ACCESSED_BY_CONTENT",
     "COMPOSITE_FRAME_ROUNDTRIP_TIME",
     "COMPOSITE_TIME",
     "CONTENT_DOCUMENTS_DESTROYED",
     "CRASH_STORE_COMPRESSED_BYTES",
     "DATABASE_LOCKED_EXCEPTION",
     "DATABASE_SUCCESSFUL_UNLOCK",
     "DATA_STORAGE_ENTRIES",
-    "DECODER_INSTANTIATED_IBM866",
-    "DECODER_INSTANTIATED_ISO2022JP",
-    "DECODER_INSTANTIATED_ISO_8859_5",
-    "DECODER_INSTANTIATED_KOI8R",
-    "DECODER_INSTANTIATED_KOI8U",
-    "DECODER_INSTANTIATED_MACARABIC",
-    "DECODER_INSTANTIATED_MACCE",
-    "DECODER_INSTANTIATED_MACCROATIAN",
-    "DECODER_INSTANTIATED_MACCYRILLIC",
-    "DECODER_INSTANTIATED_MACDEVANAGARI",
-    "DECODER_INSTANTIATED_MACFARSI",
-    "DECODER_INSTANTIATED_MACGREEK",
-    "DECODER_INSTANTIATED_MACGUJARATI",
-    "DECODER_INSTANTIATED_MACGURMUKHI",
-    "DECODER_INSTANTIATED_MACHEBREW",
-    "DECODER_INSTANTIATED_MACICELANDIC",
-    "DECODER_INSTANTIATED_MACROMANIAN",
-    "DECODER_INSTANTIATED_MACTURKISH",
     "DEFECTIVE_PERMISSIONS_SQL_REMOVED",
     "DEFERRED_FINALIZE_ASYNC",
     "DENIED_TRANSLATION_OFFERS",
     "DEVICE_RESET_REASON",
     "DEVTOOLS_ANIMATIONINSPECTOR_TIME_ACTIVE_SECONDS",
     "DEVTOOLS_BROWSERCONSOLE_TIME_ACTIVE_SECONDS",
     "DEVTOOLS_CANVASDEBUGGER_TIME_ACTIVE_SECONDS",
     "DEVTOOLS_COMPUTEDVIEW_TIME_ACTIVE_SECONDS",
@@ -752,34 +734,16 @@
     "CYCLE_COLLECTOR_WORKER_NEED_GC",
     "CYCLE_COLLECTOR_WORKER_OOM",
     "CYCLE_COLLECTOR_WORKER_VISITED_GCED",
     "CYCLE_COLLECTOR_WORKER_VISITED_REF_COUNTED",
     "D3D11_SYNC_HANDLE_FAILURE",
     "DATABASE_LOCKED_EXCEPTION",
     "DATABASE_SUCCESSFUL_UNLOCK",
     "DATA_STORAGE_ENTRIES",
-    "DECODER_INSTANTIATED_IBM866",
-    "DECODER_INSTANTIATED_ISO2022JP",
-    "DECODER_INSTANTIATED_ISO_8859_5",
-    "DECODER_INSTANTIATED_KOI8R",
-    "DECODER_INSTANTIATED_KOI8U",
-    "DECODER_INSTANTIATED_MACARABIC",
-    "DECODER_INSTANTIATED_MACCE",
-    "DECODER_INSTANTIATED_MACCROATIAN",
-    "DECODER_INSTANTIATED_MACCYRILLIC",
-    "DECODER_INSTANTIATED_MACDEVANAGARI",
-    "DECODER_INSTANTIATED_MACFARSI",
-    "DECODER_INSTANTIATED_MACGREEK",
-    "DECODER_INSTANTIATED_MACGUJARATI",
-    "DECODER_INSTANTIATED_MACGURMUKHI",
-    "DECODER_INSTANTIATED_MACHEBREW",
-    "DECODER_INSTANTIATED_MACICELANDIC",
-    "DECODER_INSTANTIATED_MACROMANIAN",
-    "DECODER_INSTANTIATED_MACTURKISH",
     "DEFECTIVE_PERMISSIONS_SQL_REMOVED",
     "DEFERRED_FINALIZE_ASYNC",
     "DENIED_TRANSLATION_OFFERS",
     "DEVICE_RESET_REASON",
     "DEVTOOLS_ABOUTDEBUGGING_TIME_ACTIVE_SECONDS",
     "DEVTOOLS_ANIMATIONINSPECTOR_TIME_ACTIVE_SECONDS",
     "DEVTOOLS_BROWSERCONSOLE_TIME_ACTIVE_SECONDS",
     "DEVTOOLS_CANVASDEBUGGER_TIME_ACTIVE_SECONDS",
@@ -1797,34 +1761,16 @@
     "CSP_DOCUMENTS_COUNT",
     "CSP_UNSAFE_EVAL_DOCUMENTS_COUNT",
     "CSP_UNSAFE_INLINE_DOCUMENTS_COUNT",
     "CYCLE_COLLECTOR_OOM",
     "CYCLE_COLLECTOR_WORKER_OOM",
     "D3D11_COMPOSITING_FAILURE_ID",
     "D3D11_SYNC_HANDLE_FAILURE",
     "D3D9_COMPOSITING_FAILURE_ID",
-    "DECODER_INSTANTIATED_IBM866",
-    "DECODER_INSTANTIATED_ISO2022JP",
-    "DECODER_INSTANTIATED_ISO_8859_5",
-    "DECODER_INSTANTIATED_KOI8R",
-    "DECODER_INSTANTIATED_KOI8U",
-    "DECODER_INSTANTIATED_MACARABIC",
-    "DECODER_INSTANTIATED_MACCE",
-    "DECODER_INSTANTIATED_MACCROATIAN",
-    "DECODER_INSTANTIATED_MACCYRILLIC",
-    "DECODER_INSTANTIATED_MACDEVANAGARI",
-    "DECODER_INSTANTIATED_MACFARSI",
-    "DECODER_INSTANTIATED_MACGREEK",
-    "DECODER_INSTANTIATED_MACGUJARATI",
-    "DECODER_INSTANTIATED_MACGURMUKHI",
-    "DECODER_INSTANTIATED_MACHEBREW",
-    "DECODER_INSTANTIATED_MACICELANDIC",
-    "DECODER_INSTANTIATED_MACROMANIAN",
-    "DECODER_INSTANTIATED_MACTURKISH",
     "DEDICATED_WORKER_SPAWN_GETS_QUEUED",
     "DEFECTIVE_PERMISSIONS_SQL_REMOVED",
     "DENIED_TRANSLATION_OFFERS",
     "DEVTOOLS_ABOUTDEBUGGING_OPENED_COUNT",
     "DEVTOOLS_ANIMATIONINSPECTOR_OPENED_COUNT",
     "DEVTOOLS_BROWSERCONSOLE_OPENED_COUNT",
     "DEVTOOLS_CANVASDEBUGGER_OPENED_COUNT",
     "DEVTOOLS_COMPUTEDVIEW_OPENED_COUNT",