Why is firefox deprecated?

The description strongly implies it’s because auto-update from within firefox does not work, but it’s not totally clear, and it doesn’t say why not auto-updating would cause a deprecation, which seems like a logical question. I scanned the linked bug report, didn’t see anything. I’m just curious.

Because it’s the last remaining binary (= app not build from source) in F-Droid mainline.

Is Firefox still supported?

The latest version in F-Droid is a little outdated.

Firefox was the last app that was not built from sources, but rather
pulled directly form upstream. It has been pending for removal for
quite some time and was move to the archive some month or weeks ago.
It was already outdated by then, but we wanted to make sure an update
description gets shipped in the index.

Feel free to use ffupdater or download directly from Mozilla – which
unfortunately still does not include automatic updates for release
builds more than two years after opening an issue ticket for this.