diff --git a/sway-portals.conf b/sway-portals.conf
index 8c3959a..2221438 100644
--- a/sway-portals.conf
+++ b/sway-portals.conf
@@ -2,3 +2,8 @@
 default=gtk
 org.freedesktop.impl.portal.ScreenCast=wlr
 org.freedesktop.impl.portal.Screenshot=wlr
+# xdg-desktop-portal-gtk's implementation uses org.gnome.SessionManager and
+# org.freedesktop.ScreenSaver, neither of which is implemented by Sway. This
+# will cause some programs (e.g. Firefox) to use Wayland's idle-inhibit
+# protocol instead, which sway does implement.
+org.freedesktop.impl.portal.Inhibit=none