Just so you know Firejail is a setuid root binary with a lot of code. This means that if that code gets exploited and the sandboxed process escapes the sandbox it will have root privileges instead of the running users.

Sources: https://github.com/netblue30/firejail, https://madaidans-insecurities.github.io/linux.html#firejail

Alright, I was able to fix it by installing it under /system/app which doesn't require any extra configuring. I'll share how I did this in an edit of the post. Thanks for all the help :)

Since most people that I know use messaging apps that nag about missing GMS I decided to get MicroG. Now, I want Signal under the Cloud Messaging hood mostly due to battery saving.

Just tried it with Aurora and I get the same message

Oh, I installed MicroG as an user app. I'll try installing it as a system app as a last resort, since reinstalling every messaging would be really tedious.

Also, one question. What do you mean by not installing it as a Magisk module? I'm not sure what other way there is, other than directly writing to /system

I reinstalled Signal from the website apk and got the same message. I currently have 5 apps registered, all displayed in the companion app.

How should I install it as a system app if not from a Magisk module?

I already fixed it

Maybe. I installed it as an user app.

It is Signal specific, all other apps, atleast on my phone, that use cloud messaging work.