diff -up firefox-92.0/dom/media/gmp/GMPChild.cpp.1725828 firefox-92.0/dom/media/gmp/GMPChild.cpp
--- firefox-92.0/dom/media/gmp/GMPChild.cpp.1725828	2021-09-01 19:14:40.000000000 +0200
+++ firefox-92.0/dom/media/gmp/GMPChild.cpp	2021-09-13 17:32:01.606739188 +0200
@@ -232,8 +232,14 @@ mozilla::ipc::IPCResult GMPChild::RecvPr
   }
 #elif defined(XP_LINUX)
   constexpr static const char* whitelist[] = {
+      // NSS libraries used by clearkey.
       "libfreeblpriv3.so",
       "libsoftokn3.so",
+      // glibc libraries merged into libc.so.6; see bug 1725828 and
+      // the corresponding code in GMPParent.cpp.
+      "libdl.so.2",
+      "libpthread.so.0",
+      "librt.so.1",
   };
 
   nsTArray<nsCString> libs;
@@ -256,7 +262,7 @@ mozilla::ipc::IPCResult GMPChild::RecvPr
           }
           // End bug 1698718 logging.
 
-          MOZ_CRASH("Couldn't load lib needed by NSS");
+          MOZ_CRASH("Couldn't load lib needed by media plugin");
         }
       }
     }
diff -up firefox-92.0/dom/media/gmp/GMPParent.cpp.1725828 firefox-92.0/dom/media/gmp/GMPParent.cpp
--- firefox-92.0/dom/media/gmp/GMPParent.cpp.1725828	2021-09-13 17:32:01.606739188 +0200
+++ firefox-92.0/dom/media/gmp/GMPParent.cpp	2021-09-13 17:55:54.902965506 +0200
@@ -873,8 +873,21 @@ RefPtr<GenericPromise> GMPParent::ParseC
     return GenericPromise::CreateAndReject(NS_ERROR_FAILURE, __func__);
   }
 
-  GMPCapability video;
+#ifdef XP_LINUX
+  // These glibc libraries were merged into libc.so.6 as of glibc
+  // 2.34; they now exist only as stub libraries for compatibility and
+  // newly linked code won't depend on them, so we need to ensure
+  // they're loaded for plugins that may have been linked against a
+  // different version of glibc.  (See also bug 1725828.)
+  if (!mDisplayName.EqualsASCII("clearkey")) {
+    if (!mLibs.IsEmpty()) {
+      mLibs.AppendLiteral(", ");
+    }
+    mLibs.AppendLiteral("libdl.so.2, libpthread.so.0, librt.so.1");
+  }
+#endif
 
+  GMPCapability video;
   nsCString codecsString = NS_ConvertUTF16toUTF8(m.mX_cdm_codecs);
   nsTArray<nsCString> codecs;
   SplitAt(",", codecsString, codecs);