@cheatyhax think of the firehose as mastodon local timeline
mobile browser i think the federated column still shows but it will just show you the same content as firehose
the federated column does show in apps for mobile so in apps you can open the federated column but no CoSo toots will show in it to other instances of mastodon
if you are logged in to a few mastodon instances via any app you can see all they toots in the federated TL as normal but again no CoSo toots will show there