Notifications aren't working in mobile browsers (Firefox and Vivaldi)

I tried to follow instructions at https://github.com/firebase/quickstart-js/tree/master/messaging as closely as possible, but the notifications aren't arriving on any mobile browser I've tested (Firefox Beta and Vivaldi) on my Android 10 phone (they're working fine on desktop).

I was able to accept permission just fine, this was also reported by friend of mine, who tested notifications for me. I tried to have browser out of focus and not running, which didn't make any difference. Web push seems to be working just fine in the same browsers (when testing https://serviceworke.rs/push-payload_demo.html)