Simple open html file button автор Siarhei Kuzeyeu

Simple button to open html file. Allow you to open local HTML page file on Android. You can choose multiply files at once. Add the extension page to your "Bookmarks" menu to open the page more convenient.

You can not just install the extension from the site. It is Mozilla's restriction. The restriction made because not all Firefox for Android functionality implemented and some extension do not work properly. But some extensions work anyway. "Simple open html file button" works fine.

You can read how to install the extension on Firefox Nightly for Android. You can not install the extension on usual Firefox for Android.
https://www.ghacks.net/2020/10/01/you-can-now-install-any-add-on-in-firefox-nightly-for-android-but-it-is-complicated/

Alternatively you can install a Firefox modification without the extension installing restriction
https://github.com/fork-maintainers/iceraven-browser