diff --git a/dom/ipc/TabParent.cpp b/dom/ipc/TabParent.cpp
index 168ee5561912..b2bd8f152d1d 100644
--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -2956,7 +2956,6 @@ public:
   NS_IMETHOD SetLoadGroup(nsILoadGroup*) NO_IMPL
   NS_IMETHOD SetLoadFlags(nsLoadFlags) NO_IMPL
   NS_IMETHOD GetLoadFlags(nsLoadFlags*) NO_IMPL
-  NS_IMETHOD GetIsDocument(bool *) NO_IMPL
   NS_IMETHOD GetOriginalURI(nsIURI**) NO_IMPL
   NS_IMETHOD SetOriginalURI(nsIURI*) NO_IMPL
   NS_IMETHOD GetURI(nsIURI** aUri) override
diff --git a/dom/jsurl/nsJSProtocolHandler.cpp b/dom/jsurl/nsJSProtocolHandler.cpp
index 1e53e93e2775..0c1b5d9599fc 100644
--- a/dom/jsurl/nsJSProtocolHandler.cpp
+++ b/dom/jsurl/nsJSProtocolHandler.cpp
@@ -442,12 +442,6 @@ nsresult nsJSChannel::Init(nsIURI* aURI, nsILoadInfo* aLoadInfo)
     return rv;
 }
 
-NS_IMETHODIMP
-nsJSChannel::GetIsDocument(bool *aIsDocument)
-{
-  return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 //
 // nsISupports implementation...
 //
diff --git a/modules/libjar/nsJARChannel.cpp b/modules/libjar/nsJARChannel.cpp
index 85383dc16cf9..7e69cb341046 100644
--- a/modules/libjar/nsJARChannel.cpp
+++ b/modules/libjar/nsJARChannel.cpp
@@ -486,12 +486,6 @@ nsJARChannel::SetLoadFlags(nsLoadFlags aLoadFlags)
     return NS_OK;
 }
 
-NS_IMETHODIMP
-nsJARChannel::GetIsDocument(bool *aIsDocument)
-{
-    return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 NS_IMETHODIMP
 nsJARChannel::GetLoadGroup(nsILoadGroup **aLoadGroup)
 {
diff --git a/netwerk/base/nsBaseChannel.cpp b/netwerk/base/nsBaseChannel.cpp
index f45a1e3b6848..42173b49d14b 100644
--- a/netwerk/base/nsBaseChannel.cpp
+++ b/netwerk/base/nsBaseChannel.cpp
@@ -503,12 +503,6 @@ nsBaseChannel::GetLoadInfo(nsILoadInfo** aLoadInfo)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-nsBaseChannel::GetIsDocument(bool *aIsDocument)
-{
-  return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 NS_IMETHODIMP
 nsBaseChannel::GetNotificationCallbacks(nsIInterfaceRequestor **aCallbacks)
 {
diff --git a/netwerk/base/nsIChannel.idl b/netwerk/base/nsIChannel.idl
index 2be8e88c3736..743e942920d2 100644
--- a/netwerk/base/nsIChannel.idl
+++ b/netwerk/base/nsIChannel.idl
@@ -208,9 +208,6 @@ interface nsIChannel : nsIRequest
     /**
      * Set (e.g., by the docshell) to indicate whether or not the channel
      * corresponds to a document URI.
-     * While setting this flag is sufficient to mark a channel as a document
-     * load, _checking_ whether the channel is a document load requires the use
-     * of the new channel.isDocument
      */
     const unsigned long LOAD_DOCUMENT_URI = 1 << 16;
 
@@ -347,26 +344,7 @@ interface nsIChannel : nsIRequest
      */
     attribute nsILoadInfo loadInfo;
 
-    /**
-     * Returns true if the channel is used to create a document.
-     * It returns true if the loadFlags have LOAD_DOCUMENT_URI set, or if
-     * LOAD_HTML_OBJECT_DATA is set and the channel has the appropriate
-     * MIME type.
-     * Note: May have the wrong value if called before OnStartRequest as we
-     * don't know the MIME type yet.
-     */
-    readonly attribute bool isDocument;
-
 %{ C++
-  inline bool IsDocument()
-  {
-    bool isDocument = false;
-    if (NS_SUCCEEDED(GetIsDocument(&isDocument)) && isDocument) {
-      return true;
-    }
-    return false;
-  }
-
   inline already_AddRefed<nsILoadInfo> GetLoadInfo()
   {
     nsCOMPtr<nsILoadInfo> result;
diff --git a/netwerk/base/nsNetUtil.cpp b/netwerk/base/nsNetUtil.cpp
index 52a0595cd955..ddc13df268e2 100644
--- a/netwerk/base/nsNetUtil.cpp
+++ b/netwerk/base/nsNetUtil.cpp
@@ -36,7 +36,6 @@
 #include "nsIMIMEHeaderParam.h"
 #include "nsIMutable.h"
 #include "nsINode.h"
-#include "nsIObjectLoadingContent.h"
 #include "nsIOfflineCacheUpdate.h"
 #include "nsIPersistentProperties2.h"
 #include "nsIPrivateBrowsingChannel.h"
@@ -358,45 +357,6 @@ NS_NewChannel(nsIChannel           **outChannel,
                                aIoService);
 }
 
-nsresult
-NS_GetIsDocumentChannel(nsIChannel * aChannel, bool *aIsDocument)
-{
-  // Check if this channel is going to be used to create a document. If it has
-  // LOAD_DOCUMENT_URI set it is trivially creating a document. If
-  // LOAD_HTML_OBJECT_DATA is set it may or may not be used to create a
-  // document, depending on its MIME type.
-
-  *aIsDocument = false;
-  if (!aChannel || !aIsDocument) {
-      return NS_ERROR_NULL_POINTER;
-  }
-  nsLoadFlags loadFlags;
-  nsresult rv = aChannel->GetLoadFlags(&loadFlags);
-  if (NS_FAILED(rv)) {
-      return rv;
-  }
-  if (loadFlags & nsIChannel::LOAD_DOCUMENT_URI) {
-      *aIsDocument = true;
-      return NS_OK;
-  }
-  if (!(loadFlags & nsIRequest::LOAD_HTML_OBJECT_DATA)) {
-      *aIsDocument = false;
-      return NS_OK;
-  }
-  nsAutoCString mimeType;
-  rv = aChannel->GetContentType(mimeType);
-  if (NS_FAILED(rv)) {
-      return rv;
-  }
-  if (nsContentUtils::HtmlObjectContentTypeForMIMEType(mimeType, nullptr) ==
-      nsIObjectLoadingContent::TYPE_DOCUMENT) {
-      *aIsDocument = true;
-      return NS_OK;
-  }
-  *aIsDocument = false;
-  return NS_OK;
-}
-
 nsresult
 NS_MakeAbsoluteURI(nsACString       &result,
                    const nsACString &spec,
diff --git a/netwerk/base/nsNetUtil.h b/netwerk/base/nsNetUtil.h
index 72cfc37e5a4f..be99db82f174 100644
--- a/netwerk/base/nsNetUtil.h
+++ b/netwerk/base/nsNetUtil.h
@@ -190,8 +190,6 @@ NS_NewChannel(nsIChannel           **outChannel,
               nsLoadFlags            aLoadFlags = nsIRequest::LOAD_NORMAL,
               nsIIOService          *aIoService = nullptr);
 
-nsresult NS_GetIsDocumentChannel(nsIChannel * aChannel, bool *aIsDocument);
-
 nsresult NS_MakeAbsoluteURI(nsACString       &result,
                             const nsACString &spec,
                             nsIURI           *baseURI);
diff --git a/netwerk/protocol/about/nsAboutCache.h b/netwerk/protocol/about/nsAboutCache.h
index 488d2c8ea820..c2d1af85054b 100644
--- a/netwerk/protocol/about/nsAboutCache.h
+++ b/netwerk/protocol/about/nsAboutCache.h
@@ -39,7 +39,6 @@
   NS_IMETHOD GetContentDispositionHeader(nsACString & aContentDispositionHeader) override { return !_to ? NS_ERROR_NULL_POINTER : _to->GetContentDispositionHeader(aContentDispositionHeader); } \
   NS_IMETHOD GetLoadInfo(nsILoadInfo * *aLoadInfo) override { return !_to ? NS_ERROR_NULL_POINTER : _to->GetLoadInfo(aLoadInfo); } \
   NS_IMETHOD SetLoadInfo(nsILoadInfo *aLoadInfo) override { return !_to ? NS_ERROR_NULL_POINTER : _to->SetLoadInfo(aLoadInfo); } \
-  NS_IMETHOD GetIsDocument(bool *aIsDocument) override { return !_to ? NS_ERROR_NULL_POINTER : _to->GetIsDocument(aIsDocument); }; \
 
 class nsAboutCache final : public nsIAboutModule
 {
diff --git a/netwerk/protocol/http/HttpBaseChannel.cpp b/netwerk/protocol/http/HttpBaseChannel.cpp
index 0742df96a633..a5e53dd2ba47 100644
--- a/netwerk/protocol/http/HttpBaseChannel.cpp
+++ b/netwerk/protocol/http/HttpBaseChannel.cpp
@@ -535,12 +535,6 @@ HttpBaseChannel::GetLoadInfo(nsILoadInfo **aLoadInfo)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-HttpBaseChannel::GetIsDocument(bool *aIsDocument)
-{
-  return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 NS_IMETHODIMP
 HttpBaseChannel::GetNotificationCallbacks(nsIInterfaceRequestor **aCallbacks)
 {
diff --git a/netwerk/protocol/http/HttpBaseChannel.h b/netwerk/protocol/http/HttpBaseChannel.h
index f11aa43ed9b9..591c08496ea8 100644
--- a/netwerk/protocol/http/HttpBaseChannel.h
+++ b/netwerk/protocol/http/HttpBaseChannel.h
@@ -135,7 +135,6 @@ public:
   NS_IMETHOD SetOwner(nsISupports *aOwner) override;
   NS_IMETHOD GetLoadInfo(nsILoadInfo **aLoadInfo) override;
   NS_IMETHOD SetLoadInfo(nsILoadInfo *aLoadInfo) override;
-  NS_IMETHOD GetIsDocument(bool *aIsDocument) override;
   NS_IMETHOD GetNotificationCallbacks(nsIInterfaceRequestor **aCallbacks) override;
   NS_IMETHOD SetNotificationCallbacks(nsIInterfaceRequestor *aCallbacks) override;
   NS_IMETHOD GetContentType(nsACString& aContentType) override;
diff --git a/netwerk/protocol/http/NullHttpChannel.cpp b/netwerk/protocol/http/NullHttpChannel.cpp
index 59da1a36b2ea..12fdb7b29347 100644
--- a/netwerk/protocol/http/NullHttpChannel.cpp
+++ b/netwerk/protocol/http/NullHttpChannel.cpp
@@ -539,12 +539,6 @@ NullHttpChannel::SetLoadFlags(nsLoadFlags aLoadFlags)
   return NS_ERROR_NOT_IMPLEMENTED;
 }
 
-NS_IMETHODIMP
-NullHttpChannel::GetIsDocument(bool *aIsDocument)
-{
-  return NS_ERROR_NOT_IMPLEMENTED;
-}
-
 //-----------------------------------------------------------------------------
 // NullHttpChannel::nsITimedChannel
 //-----------------------------------------------------------------------------
diff --git a/netwerk/protocol/http/nsIHttpChannel.idl b/netwerk/protocol/http/nsIHttpChannel.idl
index e9bf4d2f8892..c3ad1330078f 100644
--- a/netwerk/protocol/http/nsIHttpChannel.idl
+++ b/netwerk/protocol/http/nsIHttpChannel.idl
@@ -286,7 +286,7 @@ interface nsIHttpChannel : nsIChannel
 
    /** Indicates whether channel should be treated as the main one for the
     *  current document.  If manually set to true, will always remain true.  Otherwise,
-    *  will be true if LOAD_DOCUMENT_URI is set in the channel's loadflags.
+    *  will be true iff LOAD_DOCUMENT_URI is set in the channel's loadflags.
     */
     [must_use] attribute boolean isMainDocumentChannel;
 
diff --git a/netwerk/protocol/viewsource/nsViewSourceChannel.cpp b/netwerk/protocol/viewsource/nsViewSourceChannel.cpp
index 2100225fc3c7..ddf81e2b95dc 100644
--- a/netwerk/protocol/viewsource/nsViewSourceChannel.cpp
+++ b/netwerk/protocol/viewsource/nsViewSourceChannel.cpp
@@ -573,14 +573,6 @@ nsViewSourceChannel::SetLoadInfo(nsILoadInfo* aLoadInfo)
     return mChannel->SetLoadInfo(aLoadInfo);
 }
 
-NS_IMETHODIMP
-nsViewSourceChannel::GetIsDocument(bool *aIsDocument)
-{
-    NS_ENSURE_TRUE(mChannel, NS_ERROR_FAILURE);
-
-    return mChannel->GetIsDocument(aIsDocument);
-}
-
 NS_IMETHODIMP
 nsViewSourceChannel::GetNotificationCallbacks(nsIInterfaceRequestor* *aNotificationCallbacks)
 {
diff --git a/netwerk/protocol/wyciwyg/WyciwygChannelChild.cpp b/netwerk/protocol/wyciwyg/WyciwygChannelChild.cpp
index 32ffaef2c42a..28ccbdf89c75 100644
--- a/netwerk/protocol/wyciwyg/WyciwygChannelChild.cpp
+++ b/netwerk/protocol/wyciwyg/WyciwygChannelChild.cpp
@@ -493,12 +493,6 @@ WyciwygChannelChild::SetLoadInfo(nsILoadInfo* aLoadInfo)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-WyciwygChannelChild::GetIsDocument(bool *aIsDocument)
-{
-  return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 NS_IMETHODIMP
 WyciwygChannelChild::GetNotificationCallbacks(nsIInterfaceRequestor * *aCallbacks)
 {
diff --git a/netwerk/protocol/wyciwyg/nsWyciwygChannel.cpp b/netwerk/protocol/wyciwyg/nsWyciwygChannel.cpp
index d2f20b80a29e..bc341a1530c7 100644
--- a/netwerk/protocol/wyciwyg/nsWyciwygChannel.cpp
+++ b/netwerk/protocol/wyciwyg/nsWyciwygChannel.cpp
@@ -166,12 +166,6 @@ nsWyciwygChannel::GetLoadFlags(uint32_t * aLoadFlags)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-nsWyciwygChannel::GetIsDocument(bool *aIsDocument)
-{
-  return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 // nsIChannel methods:
 ///////////////////////////////////////////////////////////////////////////////
diff --git a/netwerk/streamconv/converters/nsMultiMixedConv.cpp b/netwerk/streamconv/converters/nsMultiMixedConv.cpp
index d0f62d919e26..e4e4b5c6f913 100644
--- a/netwerk/streamconv/converters/nsMultiMixedConv.cpp
+++ b/netwerk/streamconv/converters/nsMultiMixedConv.cpp
@@ -227,12 +227,6 @@ nsPartChannel::SetLoadFlags(nsLoadFlags aLoadFlags)
     return NS_OK;
 }
 
-NS_IMETHODIMP
-nsPartChannel::GetIsDocument(bool *aIsDocument)
-{
-    return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 NS_IMETHODIMP
 nsPartChannel::GetLoadGroup(nsILoadGroup* *aLoadGroup)
 {
diff --git a/uriloader/exthandler/ExternalHelperAppParent.cpp b/uriloader/exthandler/ExternalHelperAppParent.cpp
index 5bd84d70e50b..9ca7a6d88fa0 100644
--- a/uriloader/exthandler/ExternalHelperAppParent.cpp
+++ b/uriloader/exthandler/ExternalHelperAppParent.cpp
@@ -350,12 +350,6 @@ ExternalHelperAppParent::SetLoadFlags(nsLoadFlags aLoadFlags)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-ExternalHelperAppParent::GetIsDocument(bool *aIsDocument)
-{
-  return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 NS_IMETHODIMP
 ExternalHelperAppParent::GetLoadGroup(nsILoadGroup* *aLoadGroup)
 {
diff --git a/uriloader/exthandler/nsExternalProtocolHandler.cpp b/uriloader/exthandler/nsExternalProtocolHandler.cpp
index 2ab27c37d90f..3b264c2c69fd 100644
--- a/uriloader/exthandler/nsExternalProtocolHandler.cpp
+++ b/uriloader/exthandler/nsExternalProtocolHandler.cpp
@@ -245,11 +245,6 @@ NS_IMETHODIMP nsExtProtocolChannel::SetLoadFlags(nsLoadFlags aLoadFlags)
   return NS_OK;
 }
 
-NS_IMETHODIMP nsExtProtocolChannel::GetIsDocument(bool *aIsDocument)
-{
-  return NS_GetIsDocumentChannel(this, aIsDocument);
-}
-
 NS_IMETHODIMP nsExtProtocolChannel::GetContentType(nsACString &aContentType)
 {
   return NS_ERROR_NOT_IMPLEMENTED;