--- a/gfx/thebes/gfxUserFontSet.cpp
+++ b/gfx/thebes/gfxUserFontSet.cpp
@@ -631,16 +631,26 @@ gfxUserFontEntry::LoadPlatformFont(const
     // if necessary. The original data in aFontData is left unchanged.
     uint32_t saneLen;
     uint32_t fontCompressionRatio = 0;
     size_t computedSize = 0;
     const uint8_t* saneData =
         SanitizeOpenTypeData(aFontData, aLength, saneLen, fontType);
     if (!saneData) {
         mFontSet->LogMessage(this, "rejected by sanitizer");
+    } else {
+        // Check whether saneData is a known OpenType format; it might be
+        // a TrueType Collection, which OTS would accept but we don't yet
+        // know how to handle. If so, discard.
+        if (gfxFontUtils::DetermineFontDataType(saneData, saneLen) !=
+            GFX_USERFONT_OPENTYPE) {
+            mFontSet->LogMessage(this, "not a supported OpenType format");
+            free((void*)saneData);
+            saneData = nullptr;
+        }
     }
     if (saneData) {
         if (saneLen) {
             fontCompressionRatio = uint32_t(100.0 * aLength / saneLen + 0.5);
             if (fontType == GFX_USERFONT_WOFF ||
                 fontType == GFX_USERFONT_WOFF2) {
                 Telemetry::Accumulate(fontType == GFX_USERFONT_WOFF ?
                                       Telemetry::WEBFONT_COMPRESSION_WOFF :