There’s RocketChat - a FOSS Slack alternative. It works great, with one caveat - it depends on the Google Cloud Messaging for notifications. But RocketChat is also mobile-browser-friendly.
If I open it in Firefox and accept the notifications, they work, but the experience is pretty far from optimal - I usually have a lot of tabs open, so it’s a mess. Additionally, after I receive a notification, I get a big, big notification about Firefox playing sounds.
There’s WebApps, but it doesn’t support the HTML5 notifications.
Is there anything I could use instead of the mobile app for an on-par experience?