diff -up firefox-81.0/toolkit/modules/GMPInstallManager.sys.mjs.old firefox-81.0/toolkit/modules/GMPInstallManager.sys.mjs
--- firefox-81.0/toolkit/modules/GMPInstallManager.sys.mjs.old	2020-09-25 10:39:04.769458703 +0200
+++ firefox-81.0/toolkit/modules/GMPInstallManager.sys.mjs	2020-09-25 10:39:22.038504747 +0200
@@ -54,10 +54,6 @@ function getScopedLogger(prefix) {

 const LOCAL_GMP_SOURCES = [
   {
-    id: "gmp-gmpopenh264",
-    src: "chrome://global/content/gmp-sources/openh264.json",
-  },
-  {
     id: "gmp-widevinecdm",
     src: "chrome://global/content/gmp-sources/widevinecdm.json",
   },
diff --git a/toolkit/content/jar.mn b/toolkit/content/jar.mn
--- a/toolkit/content/jar.mn
+++ b/toolkit/content/jar.mn
@@ -108,7 +108,6 @@ toolkit.jar:
 #ifdef XP_MACOSX
    content/global/macWindowMenu.js
 #endif
-   content/global/gmp-sources/openh264.json    (gmp-sources/openh264.json)
    content/global/gmp-sources/widevinecdm.json (gmp-sources/widevinecdm.json)

 # Third party files
diff --git a/toolkit/modules/GMPInstallManager.sys.mjs b/toolkit/modules/GMPInstallManager.sys.mjs
--- a/toolkit/modules/GMPInstallManager.sys.mjs
+++ b/toolkit/modules/GMPInstallManager.sys.mjs
@@ -238,6 +234,9 @@ GMPInstallManager.prototype = {
    *                 downloaderr, verifyerr or previouserrorencountered
    */
   installAddon(gmpAddon) {
+    if (gmpAddon.isOpenH264) {
+      return Promise.reject({ type: "disabled" });
+    }
     if (this._deferred) {
       let log = getScopedLogger("GMPInstallManager.installAddon");
       log.error("previous error encountered");