--- a/layout/style/FontFaceSet.cpp
+++ b/layout/style/FontFaceSet.cpp
@@ -575,25 +575,50 @@ FontFaceSet::ForEach(JSContext* aCx,
 }
 
 void
 FontFaceSet::RemoveLoader(nsFontFaceLoader* aLoader)
 {
   mLoaders.RemoveEntry(aLoader);
 }
 
+#ifndef RELEASE_BUILD
+// This code is only intended for use during 55 nightly, and will be removed
+// directly thereafter
+
+void
+ProcessFontLoadTelemetry(const nsCOMPtr<nsIURI>& uri)
+{
+  nsCString path;
+  uri->GetPath(path);
+
+  const nsCaseInsensitiveUTF8StringComparator comparator = nsCaseInsensitiveUTF8StringComparator();
+  const bool standardExtension = StringEndsWith(path, NS_LITERAL_CSTRING(".woff2"), comparator) ||
+    StringEndsWith(path, NS_LITERAL_CSTRING(".woff"), comparator) ||
+    StringEndsWith(path, NS_LITERAL_CSTRING(".otf"), comparator) ||
+    StringEndsWith(path, NS_LITERAL_CSTRING(".eot"), comparator) ||
+    StringEndsWith(path, NS_LITERAL_CSTRING(".ttf"), comparator);
+
+  Telemetry::Accumulate(Telemetry::WEBFONT_STANDARD_EXTENSION, standardExtension);
+}
+#endif
+
 nsresult
 FontFaceSet::StartLoad(gfxUserFontEntry* aUserFontEntry,
                        const gfxFontFaceSrc* aFontFaceSrc)
 {
   nsresult rv;
 
   nsCOMPtr<nsIStreamLoader> streamLoader;
   nsCOMPtr<nsILoadGroup> loadGroup(mDocument->GetDocumentLoadGroup());
 
+#ifndef RELEASE_BUILD
+  ProcessFontLoadTelemetry(aFontFaceSrc->mURI);
+#endif
+
   nsCOMPtr<nsIChannel> channel;
   // Note we are calling NS_NewChannelWithTriggeringPrincipal() with both a
   // node and a principal.  This is because the document where the font is
   // being loaded might have a different origin from the principal of the
   // stylesheet that initiated the font load.
   rv = NS_NewChannelWithTriggeringPrincipal(getter_AddRefs(channel),
                                             aFontFaceSrc->mURI,
                                             mDocument,

--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -10306,16 +10306,23 @@
   },
   "WEBFONT_SRCTYPE": {
     "alert_emails": ["jdaggett@mozilla.com"],
     "expires_in_version": "never",
     "kind": "enumerated",
     "n_values": 5,
     "description": "Font src type loaded (1 = local, 2 = url, 3 = data)"
   },
+  "WEBFONT_STANDARD_EXTENSION": {
+    "expires_in_version": "56",
+    "kind": "boolean",
+    "description": "Whether a webfont being loaded has a standard extension (true for .woff/.woff2/.eot/.ttf/.svg#/.otf). This is different to FONTTYPE - a valid webfont doesn't have to have the corresponding extension as part of its path/URL.",
+    "bug_numbers": [1342479],
+    "alert_emails": ["ahunt@mozilla.com"]
+  },
   "WEBFONT_PER_PAGE": {
     "alert_emails": ["jdaggett@mozilla.com"],
     "expires_in_version": "never",
     "kind": "count",
     "description": "Number of fonts loaded at page load"
   },
   "WEBFONT_SIZE_PER_PAGE": {
     "alert_emails": ["jdaggett@mozilla.com"],