Create or use an existing autoconfig file in the main Firefox folder and paste the following:

//
var {classes:Cc,interfaces:Ci,utils:Cu} = Components;

/* set new tab page */
try {
Cu.import("resource:///modules/AboutNewTab.jsm");
var newTabURL = "file:///about:blank";
AboutNewTab.newTabURL = newTabURL;
} catch(e){Cu.reportError(e);} // report errors in the Browser Console

⚠️ Make sure to replace the "file:///about:blank" with the path to your chosen folder containing the homepage files. It should look something like this: "file:///C:/Users/John/Desktop/homepage-folder/home.html"
If you need help making a homepage, this link [www.hostinger.in] will take you to a helpful guide to get started. ⚠️

You can use the autoconfig.cfg file in the Firefox program folder to initialize (set/lock) preferences.

This also requires an autoconfig.js file in the "defaults/pref" folder where the channel-prefs.js file is located that specifies to use autoconfig.cfg.

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

The autoconfig.cfg and autoconfig.js files need to start with a comment line (//).

And you're done, now it should open to your custom made homepage when you open a new tab.

All this info was taken from this thread. [support.mozilla.org]
Many thanks to the moderator who provided the necessary help.