Note from @MichaelTunnell:
this issue is not related to DLN but rather was local device issue for the user.
Maybe the DLN Discourse server is hacked? I immediately deleted the 4Play.apk file, and didn’t install it. It was a 1.5 MB download. I’m on Lineage OS 15.1, which is effectively Android 8, patched up to Sept 25th version. (Sept security update).
The .apk download spontaneously happened while I was composing a reply in this Discourse (like 15 minutes ago)! I’m using the latest Firefox for Android, ver. 68.1.1
I have been doing some testing and I cant not see any issue on our end. We have a hardened server, discourse in a container and the IP to the server isnt that. I think its more likely that you have something on your end that just happened to activate during the time you were using the forum rather than the forum being the source. I will continue to do further testing but it looks like it isn’t related to the forum.
OK, Thanks everyone. When I went to take a screenshot of firefox’s downloads page on my phone, my screenshot capturing tool wasn’t behaving normally. So that indeed suggested my phone was to blame, and was hacked.
I’ve been keeping some phone backups (thanks, Teamwin!), so I’ve rolled back to a backup from about a week ago. The screenshot tool works again as normal, with that rolled-back version. So that suggests things are normal again on my phone.