[HowTo] Change file manager to Nemo for Mozilla Firefox (v100+)

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Follow up on this topic that is closed, to use Nemo as default file manager instead of Nautilus:

Step 1.
Set Nemo as default filemanager for the current system user by running the following commands (this will update $HOME/.config/mimeapps.list):
xdg-mime default nemo.desktop inode/directory
xdg-mime default nemo.desktop x-directory/normal
xdg-mime default nemo-autorun-software.desktop x-content/unix-software
update-desktop-database $HOME/.local/share/applications/

And also set Nemo as default file manager for root user:
sudo xdg-mime default nemo.desktop inode/directory
xdg-mime default nemo.desktop x-directory/normal
xdg-mime default nemo-autorun-software.desktop x-content/unix-software
sudo update-desktop-database /root/.local/share/applications/

Step 2.
This will only apply to the current system user and has to be set per Firefox profile:

widget.use-xdg-desktop-portal.file-picker to 1
widget.use-xdg-desktop-portal.mime-handler to 1

Alternative Step 2.
If you want to apply it for ALL current and FUTURE firefox profiles for all system users, you need to change the Firefox default values. To configure firefox default settings in general, use this script that follows Mozilla Firefox official documentation.

If you only want to create a default config with the goal of making Firefox use the default file manager, you do not need to set policies (the third action below).

#!/bin/bash
#
# Consider deleting all your firefox profiles first (not required).

# ENABLE DEFAULT CONFIG FILE:
sudo tee -a /usr/lib/firefox/defaults/pref/autoconfig.js &>/dev/null << EOF
pref("general.config.filename", "firefox.cfg");
pref("general.config.obscure_value", 0);
EOF
#
# CREATE DEFAULT CONFIG FILE
# -Use system default file manager - include toolbar layout in Sync - Enable bookmarks bar - set toolbar layout
sudo tee -a /usr/lib/firefox/firefox.cfg &>/dev/null << EOF
// IMPORTANT: Start your code on the 2nd line
defaultPref("widget.use-xdg-desktop-portal.file-picker",1);
defaultPref("widget.use-xdg-desktop-portal.mime-handler",1);
defaultPref("services.sync.prefs.sync.browser.uiCustomization.state",true);
defaultPref("browser.toolbars.bookmarks.visibility", "always");
defaultPref("browser.uiCustomization.state", "{\"placements\":{\"widget-overflow-fixed-list\":[\"screenshot-button\",\"print-button\",\"save-to-pocket-button\",\"bookmarks-menu-button\",\"library-button\",\"preferences-button\",\"panic-button\"],\"nav-bar\":[\"back-button\",\"forward-button\",\"stop-reload-button\",\"customizableui-special-spring1\",\"downloads-button\",\"ublock0_raymondhill_net-browser-action\",\"urlbar-container\",\"customizableui-special-spring2\"],\"toolbar-menubar\":[\"menubar-items\"],\"TabsToolbar\":[\"tabbrowser-tabs\",\"new-tab-button\",\"alltabs-button\"],\"PersonalToolbar\":[\"fxa-toolbar-menu-button\",\"history-panelmenu\",\"personal-bookmarks\"]},\"seen\":[\"save-to-pocket-button\",\"_d133e097-46d9-4ecc-9903-fa6a722a6e0e_-browser-action\",\"_contain-facebook-browser-action\",\"sponsorblocker_ajay_app-browser-action\",\"ublock0_raymondhill_net-browser-action\",\"developer-button\"],\"dirtyAreaCache\":[\"nav-bar\",\"widget-overflow-fixed-list\",\"PersonalToolbar\"],\"currentVersion\":17,\"newElementCount\":3}");
EOF
#
# CREATE DEFAULT POLICIES
# -Cleanup bookmarks toolbar by disabling default Mozilla bookmarks - install bare minimum extensions
sudo tee -a /usr/lib/firefox/distribution/policies.json &>/dev/null << EOF
{
  "policies": {
    "DisableProfileImport": true,
    "NoDefaultBookmarks": true,
    "Extensions": {
      "Install": ["https://addons.mozilla.org/firefox/downloads/latest/ublock-origin/latest.xpi", "https://addons.mozilla.org/firefox/downloads/latest/bypass-paywalls-clean/latest.xpi", "https://addons.mozilla.org/firefox/downloads/latest/sponsorblock/latest.xpi", "https://addons.mozilla.org/firefox/downloads/latest/facebook-container/latest.xpi", "https://addons.mozilla.org/firefox/downloads/latest/google-container/latest.xpi", "https://addons.mozilla.org/firefox/downloads/latest/nord-polar-night-theme/latest.xpi"]
    }
  }
}
EOF

Now, Nemo will be opened when you (for example) click on “open in folder” on a downloaded file.

Also see the discussion here:
1285711 - Firefox does not use default file manager

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