Izax

Try soaking it again overnight but with dishwasher detergent instead of normal dish soap

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