I have installed the flatpak version of Firefox, which is far more up to date than the ESR version in Debian’s repos.
To load a file from a local directory (e.g. a .html file created using reveal.js), I needed to give it permission to access the directory. I decided to give it permission to my /home directory:
flatpak override org.mozilla.firefox --filesystem=/home/neil/
And I restarted Firefox.