You can remove the figure - here's uBlock Origin filter to block it from loading:

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

(c) https://news.ycombinator.com/item?id=46310941

There's also

  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

Works in Thunderbird too.

uBlock Origin filter to block the anime girl from loading:

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

(c) https://news.ycombinator.com/item?id=46310941

You may restrict updates in policies.json[1][2]:

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

Syntax should be slightly different for Windows.

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

When you decide to update you may temporarily replace policy for permissive one, or you may just install new version in a separate $FIREFOX_DISTRIBUTION_DIR.

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

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

Even better, I switched to the esr release. It gets updates every week or two, but only for security issues. Otherwise the browser has a stable feature set and no new features.