diff --git a/introduction/welcome/changelog.xml b/introduction/welcome/changelog.xml
index 47997d6bda..71dfb74e0e 100644
--- a/introduction/welcome/changelog.xml
+++ b/introduction/welcome/changelog.xml
@@ -41,6 +41,17 @@
       </itemizedlist>
     </listitem>
     -->
+    <listitem>
+      <para>August 16th, 2021</para>
+      <itemizedlist>
+        <listitem>
+          <para>[ken] - Firefox-91 and Thunderbird: Replace
+          create-mach-environment by MACH_USE_SYSTEM_PYTHON=1. Fixes
+          <ulink url="&blfs-ticket-root;15405">#15405</ulink>.</para>
+        </listitem>
+      </itemizedlist>
+    </listitem>
+
     <listitem>
       <para>August 15th, 2021</para>
       <itemizedlist>
diff --git a/xsoft/graphweb/firefox.xml b/xsoft/graphweb/firefox.xml
index 9a48d8ded4..f3aeea8203 100644
--- a/xsoft/graphweb/firefox.xml
+++ b/xsoft/graphweb/firefox.xml
@@ -383,12 +383,12 @@ esac</userinput></screen>
 <screen><userinput remap="pre">patch -p1 -i ../firefox-&firefox-version;esr-rustc1470-1.patch</userinput></screen>-->
 
     <para>
-      Now invoke the Python script to compile the package.
+      Now invoke the Python <command>mach</command> script to compile the package.
     </para>
 
 <screen><userinput>export CC=gcc CXX=g++ &amp;&amp;
+export MACH_USE_SYSTEM_PYTHON=1            &amp;&amp;
 export MOZBUILD_STATE_PATH=${PWD}/mozbuild &amp;&amp;
-./mach create-mach-environment             &amp;&amp;
 ./mach configure                           &amp;&amp;
 ./mach build</userinput></screen>
 
@@ -407,13 +407,13 @@ export MOZBUILD_STATE_PATH=${PWD}/mozbuild &amp;&amp;
       Now, as the <systemitem class="username">root</systemitem> user:
     </para>
 
-<screen role="root"><userinput>./mach install</userinput></screen>
+<screen role="root"><userinput>MACH_USE_SYSTEM_PYTHON=1 ./mach install</userinput></screen>
 
     <para>
-      Set environment variables back to their values:
+      Empty the environment variables which were set above:
     </para>
 
-<screen><userinput>unset CC CXX MOZBUILD_STATE_PATH</userinput></screen>
+<screen><userinput>unset CC CXX MACH_USE_SYSTEM_PYTHON MOZBUILD_STATE_PATH</userinput></screen>
 
   </sect2>
 
@@ -456,8 +456,9 @@ export MOZBUILD_STATE_PATH=${PWD}/mozbuild &amp;&amp;
     </para>
 
     <para>
-      <command>./mach create-mach-environment</command>: This uses the system
-      python to create a virtual environment for <command>mach</command>.
+      <command>MACH_USE_SYSTEM_PYTHON=1</command>: Use the system python
+      to create a virtual environment for <command>mach</command> without
+      downloading any python wheels.
     </para>
 
     <para>
@@ -465,12 +466,6 @@ export MOZBUILD_STATE_PATH=${PWD}/mozbuild &amp;&amp;
       dependencies and the <filename>mozconfig</filename>.
     </para>
 
-    <para>
-      <command>./mach build</command>: <application>Firefox</application>
-      now uses this <application>python</application> script to run the
-      build and install.
-    </para>
-
     <para>
       <option>./mach build --verbose</option>: Use this alternative if you
       need details of which files are being compiled, together with any C or
diff --git a/xsoft/other/thunderbird.xml b/xsoft/other/thunderbird.xml
index 47e9308129..dd130709da 100644
--- a/xsoft/other/thunderbird.xml
+++ b/xsoft/other/thunderbird.xml
@@ -222,8 +222,8 @@ EOF</userinput></screen>
 <screen><userinput remap="pre">patch -p1 -i ../thunderbird-&thunderbird-version;-glibc234-1.patch</userinput></screen>
 
     <para>
-      Compile <application>Thunderbird</application> by issuing the following
-      commands:
+      Now invoke the Python <command>mach</command> script to compile
+      <application>Thunderbird</application>:
     </para>
 
     <note>
@@ -236,6 +236,7 @@ EOF</userinput></screen>
     </note>
 
 <screen><userinput>export CC=gcc CXX=g++ &amp;&amp;
+export MACH_USE_SYSTEM_PYTHON=1 &amp;&amp;
 ./mach create-mach-environment &amp;&amp;
 ./mach configure      &amp;&amp;
 ./mach build</userinput></screen>
@@ -249,7 +250,13 @@ EOF</userinput></screen>
       commands as the <systemitem class="username">root</systemitem> user:
     </para>
 
-<screen role="root"><userinput>./mach install</userinput></screen>
+<screen role="root"><userinput>MACH_USE_SYSTEM_PYTHON=1 ./mach install</userinput></screen>
+
+    <para>
+      Empty the environment variables which were set above:
+    </para>
+
+<screen><userinput>unset CC CXX MACH_USE_SYSTEM_PYTHON</userinput></screen>
 
   </sect2>
 
@@ -274,6 +281,12 @@ EOF</userinput></screen>
       significantly smaller build time.-->
     </para>
 
+    <para>
+      <command>MACH_USE_SYSTEM_PYTHON=1</command>: Use the system python
+      to create a virtual environment for <command>mach</command> without
+      downloading any python wheels.
+    </para>
+
     <para>
       <option>./mach build --verbose</option>: Use this alternative if you
       need details of which files are being compiled, together with any C or