It also supports about:config settings with the AutoConfig feature. Does this help? https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig

I would just go with Debian personally since it's not updated very often.

• You can add Firefox to startup applications several ways, such as startup apps GUI on any desktop environment.
• Most desktop environments have an "automatic login" option you can configure. You can also disable or remove the login manager
• You can configure some desktop environments (xfce I know for sure) to not launch the panel. Don't forget to also look up how to disable your TTY hotkeys!
• Install the unattended-upgrades package and Debian/Ubuntu will update automatically. I hardly ever have to touch the updates on my Debian machines
• You can make Firefox's config directory read only and it won't be able to write to it

Nice to haves:

• Plymouth can be installed on Debian, and has a few themes that are simple spinners
• You can do this. Look up how to do the policies.json file. Since the Firefox directory will be read only, it won't be able to be changed by the user
• I think you can still install add-ons like an ad blocker with policies.json, but not 100% dure
• Do a preseed install or similar https://wiki.debian.org/DebianInstaller/Preseed

And now it looks like a gas station bathroom!

I didn't say Abe Lincoln I said Hey Blinkin

Nah. As long as your instance is federating with all the ones that you want to interact with, there isn't much of a reason to have multiple accounts. However, if you want to see and interact with content that your main instance has blocked, you might then make another account.

Sync is closed source, while most of the other apps are open source. So you can't see or modify the code of Sync.

I'll keep using Firefox until I'm dead or it's dead!