--- a/browser/config/mozconfigs/linux64/add-on-devel
+++ b/browser/config/mozconfigs/linux64/add-on-devel
@@ -1,9 +1,12 @@
 . $topsrcdir/browser/config/mozconfigs/linux64/nightly
 
 #add-on signing is checked but not enforced
 MOZ_REQUIRE_SIGNING=0
+# Enable MOZ_ALLOW_LEGACY_EXTENSIONS
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
+
 ac_add_options --with-branding=browser/branding/unofficial
 ac_add_options --enable-update-channel=default
 
 # Need this to prevent name conflicts with the normal nightly build packages
 export MOZ_PKG_SPECIAL=add-on-devel

--- a/browser/config/mozconfigs/macosx64/add-on-devel
+++ b/browser/config/mozconfigs/macosx64/add-on-devel
@@ -1,9 +1,12 @@
 . $topsrcdir/browser/config/mozconfigs/macosx64/nightly
 
 #add-on signing is checked but not enforced
 MOZ_REQUIRE_SIGNING=0
+# Enable MOZ_ALLOW_LEGACY_EXTENSIONS
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
+
 ac_add_options --with-branding=browser/branding/unofficial
 ac_add_options --enable-update-channel=default
 
 # Need this to prevent name conflicts with the normal nightly build packages
 export MOZ_PKG_SPECIAL=add-on-devel

--- a/browser/config/mozconfigs/win32/add-on-devel
+++ b/browser/config/mozconfigs/win32/add-on-devel
@@ -1,9 +1,12 @@
 . $topsrcdir/browser/config/mozconfigs/win32/nightly
 
 #add-on signing is checked but not enforced
 MOZ_REQUIRE_SIGNING=0
+# Enable MOZ_ALLOW_LEGACY_EXTENSIONS
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
+
 ac_add_options --with-branding=browser/branding/unofficial
 ac_add_options --enable-update-channel=default
 
 # Need this to prevent name conflicts with the normal nightly build packages
 export MOZ_PKG_SPECIAL=add-on-devel

--- a/browser/config/mozconfigs/win64/add-on-devel
+++ b/browser/config/mozconfigs/win64/add-on-devel
@@ -1,9 +1,12 @@
 . $topsrcdir/browser/config/mozconfigs/win64/nightly
 
 #add-on signing is checked but not enforced
 MOZ_REQUIRE_SIGNING=0
+# Enable MOZ_ALLOW_LEGACY_EXTENSIONS
+ac_add_options "MOZ_ALLOW_LEGACY_EXTENSIONS=1"
+
 ac_add_options --with-branding=browser/branding/unofficial
 ac_add_options --enable-update-channel=default
 
 #Need this to prevent name conflicts with the normal nightly build packages
 export MOZ_PKG_SPECIAL=add-on-devel