Firefox fails to send native notifications

I have a new computer, but I fail to get native notifications to work with Firefox. It works fine on the old machine. notify-send and Google Chrome work fine on the new machine, too.

I have installed sway (window manager) and mako via apt. libnotify4 is installed, too. mako is running. But when I send a notification (from https://www.bennish.net/web-notifications.html), the notification is displayed by Firefox directly (which is super annoying with a tiling window manager). This message is printed to the console:

(firefox:78314): libnotify-WARNING **: 18:23:58.963: Failed to connect to proxy

Could it be blocked by snap or apparmor? As I said, all other apps can send notifications just fine, so it must be a problem with my Firefox.

Purging and reinstalling Firefox didn't help.