widget.gtk.overlay-scrollbars.enabled: false - always show normal scrollbar, don't just expand on hover
  widget.non-native-theme.scrollbar.style: (it seems that values 1-5 are allowed)
  widget.non-native-theme.scrollbar.size.override: width

  ! Title: Hide Anubis Image
  */.within.website/x/cmd/anubis/static/img/*.webp$image

  {
    "policies": {
      "AppUpdateURL": "http://127.0.0.1/",
      "DisableAppUpdate": true,
      "DisableFirefoxStudies": true,
      "DisableSystemAddonUpdate": true,
      "DontCheckDefaultBrowser": true,
      "ExtensionUpdate": false,
    }
  }

File should be located in $FIREFOX_DISTRIBUTION_DIR/distribution/

$FIREFOX_DISTRIBUTION_DIR is the installation directory for firefox, which (for default setup) probably will be printed by this:

  sed -nE 's/^\s*exec (.*\/)firefox .*/\1/p' $(which firefox) # and it's probably /usr/lib/firefox

[1] https://firefox-admin-docs.mozilla.org/guides/policies-confi...

[2] https://firefox-admin-docs.mozilla.org/reference/policies/

You can start with this page[4] for an examples of simple, but elegant styling.

And /r/FirefoxCSS can demonstrate all kinds of crazy options userChome.css enthusiasts can come up with.

[1] https://www.userchrome.org/

[2] https://kb.mozillazine.org/index.php?title=UserChrome.css

[3] https://old.reddit.com/r/FirefoxCSS/wiki/index/tutorials

[4] https://www.userchrome.org/firefox-89-styling-proton-ui.html...

  ! Title: Hide Anubis Image
  */.within.website/x/cmd/anubis/static/img/*.webp$image

- Independent set of settings/about:config parameters.

You can't turn off, say, WebRTC completely for some profiles, while allowing it for other profiles.

Different history. I remember accidentally nuking history of a few years - that wasn't fun. Now, you reduce blast radius.

Proxy on/off or different proxies. Though, there's probably an extension that manages it on per-site basis.

Different userChrome.css, if you fancy that.

I'd argue, that for some, CLI path is actually cleaner.

You see, the way described above creates entirely separate points of entry, and you don't have to go to the central menu to launch specific profile.

It eliminates one step (Profile Manager, about:profiles or whatever) allowing you to get faster to the desired profile - same way you'd launch a default profile.

It's logical separation too. It's like separate browsers from UX standpoint (they do use the same distribution though ...unless they aren't - you can configure different distributions for different profiles - nothing stops you from that).

I think this was as recent as 25 years ago?

Recently they added some new UI. There was and still is (I think) classic Profile Manager UI, which you can launch with

  ./firefox -ProfileManager

But you don't have to use any of those anyway - see my comment above (a response to parent).

Firefox with a non-default profile can be created like that:

  ./firefox -CreateProfile "profile-name /home/user/.mozilla/firefox/profile-dir/"
  # For, say, cloudflare that would be:
  ./firefox -CreateProfile "cloudflare /home/user/.mozilla/firefox/cloudflare/"

  ./firefox -profile "/home/user/.mozilla/firefox/profile-dir/"
  # For cloudflare that would be:
  ./firefox -profile "/home/user/.mozilla/firefox/cloudflare/"

    - create a copy of it, say, /usr/bin/firefox-cloudflare
    - adjust the relevant line, adding the -profile argument

Of course, "./firefox" from examples above should be replaced with the actual path to executable. For default installation of Firefox the path would be in /usr/bin/firefox script.

So, you can have a separate profiles for something sensitive/invasive (linkedin, cloudflare, shops, banks, etc.) and then you can have a separate profile for everything else.

And each profile can have its own set of extensions.

Tampermonkey has analytics enabled, and is closed source.

Here's a comprehensive summary on all three. [3]

[1] https://addons.mozilla.org/en-US/firefox/addon/violentmonkey

[2] https://violentmonkey.github.io/

[3] https://news.ycombinator.com/item?id=26057106

Firefox with a non-default profile can be created like that:

  ./firefox -CreateProfile "profile-name /home/user/.mozilla/firefox/profile-dir/"
  # For linkedin that would be:
  ./firefox -CreateProfile "linkedin /home/user/.mozilla/firefox/linkedin/"

  ./firefox -profile "/home/user/.mozilla/firefox/profile-dir/"
  # For linkedin that would be:
  ./firefox -profile "/home/user/.mozilla/firefox/linkedin/"

    - create a copy of it, say, /usr/bin/firefox-linkedin
    - adjust the relevant line, adding the -profile argument

Of course, "./firefox" from examples above should be replaced with the actual path to executable. For default installation of Firefox the path would be in /usr/bin/firefox script.

So, you can have a separate profiles for something sensitive/invasive (linkedin, shops, etc.) and then you can have a separate profile for everything else.

And each profile can have its own set of extensions.

    Night Filter is an easy-to-use screen filter app for your Android device. Adjust brightness and colour, set up schedules, and reduce eye strain for comfortable night-time reading. [1]

You can store your custom preferences in user.js file - Firefox will copy those to prefs.js at startup.

From your link:

   The user.js[1] file is optional. If you have one whenever the application is started it will overwrite any settings in prefs.js with the corresponding settings from user.js. 

    geo.provider.use_corelocation: true/false # presumably for tracking on MacOS

    geo.provider.use_geoclue: true/false # presumably for tracking Linux users with Geoclue2 provider [1]

    geo.enabled: true/false # presumably, turns the whole thing off

It also appears[3][4], that setting geo.provider.testing to true might be required.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1063572

[2] https://stackoverflow.com/questions/24932199/how-to-change-f...

[3] https://security.stackexchange.com/a/268825

[4] https://stackoverflow.com/a/24937564

    ./firefox -CreateProfile "profile-name /home/user/.mozilla/firefox/profile-path/"
    ./firefox -profile "/home/user/.mozilla/firefox/profile-path/"

Given that /usr/bin/firefox is just a shell script, you can

    - create a copy of it, say, /usr/bin/firefox-hn
    - adjust the relevant line, adding the -profile argument

    random_string + '.' + "profile-name"

If you want predictable, non-random directory names, you'll have to specify it.

No need to go to about:profiles. Use a separate shortcut with a -profile option added.

You store browser preferences in it and firefox copies said preferences to prefs.js on startup (overwriting on conflict).

    ./firefox -CreateProfile "profile-name /home/user/.mozilla/firefox/profile-path/"

    ./firefox -profile "/home/user/.mozilla/firefox/profile-path/"

Disclaimer: I don't use macOS, but I'm sure it's similar to Linux in that regard. You'll need to change paths, of course.

   ./firefox -CreateProfile "profile-name /home/user/.mozilla/firefox/profile-path/"