--- a/mail/config/mozconfigs/linux32/l10n-mozconfig
+++ b/mail/config/mozconfigs/linux32/l10n-mozconfig
@@ -26,8 +26,12 @@ MOZ_ADDON_SIGNING=0
 # Disable enforcing that add-ons are signed by the trusted root
 MOZ_REQUIRE_ADDON_SIGNING=0
 
 ac_add_options --disable-stdcxx-compat
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-l10n-args ] && cat $topsrcdir/build/client.py-l10n-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/linux32/nightly
+++ b/mail/config/mozconfigs/linux32/nightly
@@ -28,8 +28,12 @@ MOZ_REQUIRE_ADDON_SIGNING=0
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-args ] && cat $topsrcdir/build/client.py-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
 
 . "$topsrcdir/build/mozconfig.cache"
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/linux64/l10n-mozconfig
+++ b/mail/config/mozconfigs/linux64/l10n-mozconfig
@@ -26,8 +26,12 @@ MOZ_ADDON_SIGNING=0
 # Disable enforcing that add-ons are signed by the trusted root
 MOZ_REQUIRE_ADDON_SIGNING=0
 
 ac_add_options --disable-stdcxx-compat
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-l10n-args ] && cat $topsrcdir/build/client.py-l10n-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/linux64/nightly
+++ b/mail/config/mozconfigs/linux64/nightly
@@ -28,8 +28,12 @@ MOZ_REQUIRE_ADDON_SIGNING=0
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-args ] && cat $topsrcdir/build/client.py-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
 
 . "$topsrcdir/build/mozconfig.cache"
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/macosx64/l10n-mozconfig
+++ b/mail/config/mozconfigs/macosx64/l10n-mozconfig
@@ -13,8 +13,12 @@ ac_add_options "MOZ_ALLOW_LEGACY_EXTENSI
 # Disable checking that add-ons are signed by the trusted root
 MOZ_ADDON_SIGNING=0
 # Disable enforcing that add-ons are signed by the trusted root
 MOZ_REQUIRE_ADDON_SIGNING=0
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-l10n-args ] && cat $topsrcdir/build/client.py-l10n-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/macosx64/nightly
+++ b/mail/config/mozconfigs/macosx64/nightly
@@ -35,8 +35,12 @@ MOZ_REQUIRE_ADDON_SIGNING=0
 ac_add_options --with-ccache
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-args ] && cat $topsrcdir/build/client.py-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
 
 # XXX hack to work around dsymutil failing on OSX builds (bug 1380381, 1382262)
 ac_add_options RUSTFLAGS='-C debuginfo=1'
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/win32/l10n-mozconfig
+++ b/mail/config/mozconfigs/win32/l10n-mozconfig
@@ -29,8 +29,12 @@ export INCLUDE=$INCLUDE:/c/Office\ 2010\
 mk_export_correct_style INCLUDE
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-l10n-args ] && cat $topsrcdir/build/client.py-l10n-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
 
 # Use the TOOLTOOL provided xz for mar generation.
 mk_add_options "export XZ=$TOOLTOOL_DIR/xz/xz.exe"
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/win32/nightly
+++ b/mail/config/mozconfigs/win32/nightly
@@ -32,8 +32,12 @@ export INCLUDE=$INCLUDE:/c/Office\ 2010\
 mk_export_correct_style INCLUDE
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-args ] && cat $topsrcdir/build/client.py-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
 
 # Use the TOOLTOOL provided xz for mar generation.
 mk_add_options "export XZ=$TOOLTOOL_DIR/xz/xz.exe"
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/win64/l10n-mozconfig
+++ b/mail/config/mozconfigs/win64/l10n-mozconfig
@@ -28,8 +28,12 @@ export INCLUDE=$INCLUDE:/c/Office\ 2010\
 mk_export_correct_style INCLUDE
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-args ] && cat $topsrcdir/build/client.py-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
 
 # Use the TOOLTOOL provided xz for mar generation.
 mk_add_options "export XZ=$TOOLTOOL_DIR/xz/xz.exe"
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"

--- a/mail/config/mozconfigs/win64/nightly
+++ b/mail/config/mozconfigs/win64/nightly
@@ -32,8 +32,12 @@ export INCLUDE=$INCLUDE:/c/Office\ 2010\
 mk_export_correct_style INCLUDE
 
 # Run client.py
 mk_add_options CLIENT_PY_ARGS="$([ -f $topsrcdir/build/client.py-args ] && cat $topsrcdir/build/client.py-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1
 
 # Use the TOOLTOOL provided xz for mar generation.
 mk_add_options "export XZ=$TOOLTOOL_DIR/xz/xz.exe"
+
+# Bug 1387239
+# Use bzip2 mar update compression, to allow upgrading of older nightly builds.
+mk_add_options "export MAR_OLD_FORMAT=1"