Thank you, guys!
i remember writing a blog post on using the Android variant for chrome, but i don't think i ever created an example for it. And i think it was only for packaged apps at that time.
Yes, previously it worked just with chrome extensions and applications.
I've actually been looking at Web Push a little lately and i noticed that to send a payload, there are some extra bits of info, that are attached to the subscription object that would probably need to be stored on the UPS somehow. I believe this is the same for FF too.
Chrome does not support payload in push notifications because they haven't implemented encryption yet. Encryption of push messages is mandatory. That's why they decided to keep it null. But according to the MDN, FF should support it [1]. I will check.
I also want to notice, that FCM/GCM does not support Web Push protocol now. Googlers just implemented a Push API [2] in Chrome (browser part of WebPush) and use their proprietary protocol for push message delivering.