Comment by dessant

I believe it is essential for Firefox to allow installing extensions in a private way that does not force us to upload our source code to Mozilla, especially considering their renewed push for user privacy.

It could be a feature that can only be enabled from the browser UI, with appropriate warnings, but the choice must be offered for those who need it.

Until that happens, those with private extensions must resort to compiling their own browser, use unbranded or developer builds, or patch Firefox at runtime to disable signature checks.

This is how you allow unsigned extensions in Firefox on Arch Linux, the same files can be edited on Windows and macOS, restart the browser after changes:

  sudo tee /usr/lib/firefox/defaults/pref/config-prefs.js &>/dev/null <<EOF
  pref("general.config.obscure_value", 0);
  pref("general.config.filename", "config.js");
  pref("general.config.sandbox_enabled", false);
  EOF

  sudo tee /usr/lib/firefox/config.js &>/dev/null <<EOF
  // keep this comment
  try {
    Components.utils
      .import('resource://gre/modules/addons/XPIDatabase.jsm', {})
      .XPIDatabase['SIGNED_TYPES'].clear();
  } catch (ex) {
    Components.utils.reportError(ex.message);
  }
  EOF