--- a/python/l10n/fluent_migrations/bug_1419547_preferences_containers.py
+++ b/python/l10n/fluent_migrations/bug_1419547_preferences_containers.py
@@ -113,19 +113,22 @@ def migrate(ctx):
                             'containers.updateContainerTitle',
                             {
                                 '%S': EXTERNAL_ARGUMENT('name')
                             },
                         ),
                     ),
                     FTL.Attribute(
                         FTL.Identifier('style'),
-                        COPY(
-                            'browser/chrome/browser/preferences/containers.dtd',
-                            'window.width',
+                        CONCAT(
+                            FTL.TextElement('width: '),
+                            COPY(
+                                'browser/chrome/browser/preferences/containers.dtd',
+                                'window.width',
+                            ),
                         ),
                     ),
                 ],
             ),
             FTL.Message(
                 id=FTL.Identifier('containers-window-close'),
                 attributes=[
                     FTL.Attribute(