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diff -up firefox-81.0/toolkit/modules/GMPInstallManager.jsm.old firefox-81.0/toolkit/modules/GMPInstallManager.jsm
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--- firefox-81.0/toolkit/modules/GMPInstallManager.jsm.old	2020-09-25 10:39:04.769458703 +0200
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+++ firefox-81.0/toolkit/modules/GMPInstallManager.jsm	2020-09-25 10:39:22.038504747 +0200
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@@ -54,10 +54,6 @@ function getScopedLogger(prefix) {
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 const LOCAL_GMP_SOURCES = [
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   {
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-    id: "gmp-gmpopenh264",
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-    src: "chrome://global/content/gmp-sources/openh264.json",
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-  },
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-  {
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     id: "gmp-widevinecdm",
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     src: "chrome://global/content/gmp-sources/widevinecdm.json",
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   },
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diff --git a/toolkit/content/jar.mn b/toolkit/content/jar.mn
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--- a/toolkit/content/jar.mn
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+++ b/toolkit/content/jar.mn
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@@ -108,7 +108,6 @@ toolkit.jar:
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 #ifdef XP_MACOSX
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    content/global/macWindowMenu.js
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 #endif
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-   content/global/gmp-sources/openh264.json    (gmp-sources/openh264.json)
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    content/global/gmp-sources/widevinecdm.json (gmp-sources/widevinecdm.json)
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 # Third party files
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diff --git a/toolkit/modules/GMPInstallManager.jsm b/toolkit/modules/GMPInstallManager.jsm
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--- a/toolkit/modules/GMPInstallManager.jsm
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+++ b/toolkit/modules/GMPInstallManager.jsm
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@@ -238,6 +234,9 @@ GMPInstallManager.prototype = {
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    *                 downloaderr, verifyerr or previouserrorencountered
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    */
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   installAddon(gmpAddon) {
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+    if (gmpAddon.isOpenH264) {
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+      return Promise.reject({ type: "disabled" });
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+    }
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     if (this._deferred) {
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       let log = getScopedLogger("GMPInstallManager.installAddon");
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       log.error("previous error encountered");
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