--- a/mail/config/mozconfigs/linux32/debug
+++ b/mail/config/mozconfigs/linux32/debug
@@ -10,16 +10,17 @@ ac_add_options --enable-calendar
 . "$topsrcdir/build/mozconfig.stylo"
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/linux32/l10n-mozconfig
+++ b/mail/config/mozconfigs/linux32/l10n-mozconfig
@@ -15,16 +15,17 @@ ac_add_options --with-l10n-base=../../l1
 ac_add_options --enable-calendar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 ac_add_options --disable-stdcxx-compat
 
 # Run client.py

--- a/mail/config/mozconfigs/linux32/nightly
+++ b/mail/config/mozconfigs/linux32/nightly
@@ -14,16 +14,17 @@ ac_add_options --enable-calendar
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/linux32/release
+++ b/mail/config/mozconfigs/linux32/release
@@ -13,16 +13,17 @@ ac_add_options --enable-calendar
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/linux64/debug
+++ b/mail/config/mozconfigs/linux64/debug
@@ -10,16 +10,17 @@ ac_add_options --enable-calendar
 . "$topsrcdir/build/mozconfig.stylo"
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/linux64/l10n-mozconfig
+++ b/mail/config/mozconfigs/linux64/l10n-mozconfig
@@ -15,16 +15,17 @@ ac_add_options --with-l10n-base=../../l1
 ac_add_options --enable-calendar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 ac_add_options --disable-stdcxx-compat
 
 # Run client.py

--- a/mail/config/mozconfigs/linux64/nightly
+++ b/mail/config/mozconfigs/linux64/nightly
@@ -14,16 +14,17 @@ ac_add_options --enable-calendar
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/linux64/release
+++ b/mail/config/mozconfigs/linux64/release
@@ -16,16 +16,17 @@ export MOZILLA_OFFICIAL=1
 export MOZ_TELEMETRY_REPORTING=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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-args ] && cat $topsrcdir/build/client.py-args)"
 mk_add_options ALWAYS_RUN_CLIENT_PY=1

--- a/mail/config/mozconfigs/macosx64/debug
+++ b/mail/config/mozconfigs/macosx64/debug
@@ -5,16 +5,17 @@ ac_add_options --enable-debug
 ac_add_options --enable-calendar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/macosx64/l10n-mozconfig
+++ b/mail/config/mozconfigs/macosx64/l10n-mozconfig
@@ -4,16 +4,17 @@ ac_add_options --disable-install-strip
 ac_add_options --enable-profiling
 ac_add_options --with-l10n-base=../../l10n
 # Disabling is needed, see bug 1345422 comment #58.
 ac_add_options --disable-compile-environment
 
 # Build lightning locales
 ac_add_options --enable-calendar
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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

--- a/mail/config/mozconfigs/macosx64/nightly
+++ b/mail/config/mozconfigs/macosx64/nightly
@@ -21,16 +21,17 @@ export MOZ_DEBUG_SYMBOLS=1
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 ac_add_options --with-ccache
 
 # Run client.py

--- a/mail/config/mozconfigs/macosx64/release
+++ b/mail/config/mozconfigs/macosx64/release
@@ -17,16 +17,17 @@ ac_add_options --enable-calendar
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/win32/debug
+++ b/mail/config/mozconfigs/win32/debug
@@ -7,16 +7,17 @@ ac_add_options --enable-debug
 ac_add_options --enable-calendar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/win32/l10n-mozconfig
+++ b/mail/config/mozconfigs/win32/l10n-mozconfig
@@ -11,16 +11,17 @@ ac_add_options --with-l10n-base=../../l1
 ac_add_options --enable-calendar
 
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 . $topsrcdir/build/win32/mozconfig.vs-latest
 
 # Set up mapi includes (must be done after visual studio setup)

--- a/mail/config/mozconfigs/win32/nightly
+++ b/mail/config/mozconfigs/win32/nightly
@@ -11,16 +11,17 @@ ac_add_options --enable-calendar
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/win32/release
+++ b/mail/config/mozconfigs/win32/release
@@ -11,16 +11,17 @@ ac_add_options --enable-calendar
 # For NSS symbols
 export MOZ_DEBUG_SYMBOLS=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 

--- a/mail/config/mozconfigs/win64/debug
+++ b/mail/config/mozconfigs/win64/debug
@@ -10,16 +10,17 @@ ac_add_options --enable-debug
 ac_add_options --enable-calendar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 . $topsrcdir/build/win64/mozconfig.vs-latest
 
 # Set up mapi includes (must be done after visual studio setup)

--- a/mail/config/mozconfigs/win64/l10n-mozconfig
+++ b/mail/config/mozconfigs/win64/l10n-mozconfig
@@ -10,16 +10,17 @@ ac_add_options --enable-update-channel=$
 # ac_add_options --disable-compile-environment
 
 # Build lightning locales
 ac_add_options --enable-calendar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 . $topsrcdir/build/win64/mozconfig.vs-latest
 
 # Set up mapi includes (must be done after visual studio setup)

--- a/mail/config/mozconfigs/win64/nightly
+++ b/mail/config/mozconfigs/win64/nightly
@@ -11,16 +11,17 @@ ac_add_options --enable-jemalloc
 ac_add_options --enable-profiling
 ac_add_options --enable-calendar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
 # 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
 
 # Package js shell
 export MOZ_PACKAGE_JSSHELL=1