You can consider to install the PrefBar extension or another extension that allows to create a button with a shortcut key.

• PrefBar: https://addons.mozilla.org/firefox/addon/prefbar/

You can use this command to open the Preferences window:

openPreferences();

Thank you for your suggestion, albeit I'm not following it, as I've decided to rely less and less on addons, as they may become obsolete, or Firefox could start again to perform poorly or - worse - to crash like there is no tomorrow.

I've found this workaround: opening the Preferences dialog and keeping it open, as it is a non-blocking dialog.

Have a nice day.