I no longer remember all the exact steps I did but I only googled them in the first place so presumably they are there to be googled still. But it's possible to fully remove snapd and all snap support and then taboo it so that it never comes back. Or at least, it's been a few years and it hasn't come back. FF has remained a real .deb from the mozillateam ppa. It was a few different steps though not just uninstalling a few packages but also editing some apt config files I think. Sorry that sounds useless but like I say I just googled it up at the time, did 15-20 minutes of reading and poking, and never had to touch it again since then. It's been several version bumps.

..edit.. I installed a dummy package that displaces the nagware about the pro version too so I never get those messages during apt update any more.

Taking a quick definitely incomplete look I see at least:

/etc/apt/preferences.d/mozilla.pref

  Package: firefox*
  Pin: release o=LP-PPA-mozillateam
  Pin-Priority: 501
  
  Package: thunderbird*
  Pin: release o=LP-PPA-mozillateam
  Pin-Priority: 501

/etc/apt/preferences.d/nosnap.pref

  Package: snapd
  Pin: release a=*
  Pin-Priority: -10

and removed ubuntu-pro-esm-apps and ubuntu-pro-esm-infra from that same dir

but also there is a mozillateam ppa in sources.list.d, and I don't see any installed package name that looks like it might be that dummy ubuntu-pro-esm thing, so maybe it got removed during a version upgrade and I never noticed because ubuntu stopped that nonsense and it isn't needed any more? Or there is some other config somewhere I'm forgetting that is keeping that hole plugged.

Anyway, it WAS a little bit of fiddling around one day, but at least it was only a one and done thing so far.

I kind of expected to be off of ubuntu by now because once someone starts doing anything like that, it doesn't matter if you can work around it, the real problem is that they want to do things like that at all in the first place. Well they still want what they want and that problem is never going away. They will just keep trying some other thing and then some other thing. So rather that fight them forever, it's better to find someone else who you don't want to fight. I mean that's why we're on Linux at all in the first place right? But so far it's been a few version bumps since then and still more or less fine.

I recommend downloading the executable-in-a-tarball form of Firefox and running that. I personally do that with Nightly, and I find it works quite well.

As someone else says, for Firefox (and Thunderbird) I just uninstalled the package manager version entirely and dropped Mozilla's regular distro-agnostic binary tarballs in my home folder. Using the built-in update systems also avoids that problem from .deb versions where updating the package could make the browser yell at you that it needs to be restarted when you try to open a tab.

No, hasn't happened so far.

Linux Mint is Ubuntu

You are talking about Debian stable which is released approximately once in 2 years. People who want to have more (most ?) recent software on Debian should go for Debian Testing. Or Debian Sid, which gets upstream updates almost instantly but requires more Linux knowledge in case something gets broken.