Have you tried this one? I don't know whether it will do what you want.

https://addons.mozilla.org/firefox/addon/new-tab-override

Have you tried this one? I don't know whether it will do what you want. https://addons.mozilla.org/firefox/addon/new-tab-override

I have tried that one, but it doesn't work. Trying to link directly to the file gives this message "Due to Firefox restrictions, it is not possible for extensions to access local files." and if I just set it to my current homepage I get "Only home pages starting with http:// or https:// are supported." which doesn't work for me because my homepage is a local html file. If I do use the local file option, it partially loads, just without any of the CSS so I just get a white screen with a few hyperlinks.

You can possibly do this directly via autoconfig.cfg and autoconfig.js.

• https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig

This code still works for me.

• /questions/1401382 Blank new tab when using file:/// for homepage with newtabhomepage addon

Firefox appears to 'esmify' automatically in case of known .jsm files that have been converted.

• https://searchfox.org/mozilla-release/source/tools/esmify/import-to-import_esmodule.js#59
• https://searchfox.org/mozilla-release/source/tools/esmify/is-esmified.js
• https://searchfox.org/mozilla-release/source/tools/esmify/map.json

Content of autoconfig.cfg:

// autoconfig.cfg start with a comment line; autoconfig.js => pref("general.config.sandbox_enabled", false);
try {
  Components.utils.import("resource:///modules/AboutNewTab.jsm");
  AboutNewTab.newTabURL = "file:///xxx";
} catch(e){Components.utils.reportError(e);} // Browser Console

Content of autoconfig.js:

//
pref("general.config.filename", "autoconfig.cfg");
pref("general.config.obscure_value", 0);
pref("general.config.sandbox_enabled", false);