Since the settings haven't given that option for years, the best way to have a local file as the new tab page was using autoconfig.cfg + autoconfig.js, as described in this thread: https://support.mozilla.org/en-US/questions/1251199 But since the most recent update (v136.0), this stopped working. The New Tab Override add-on has its own issues with using local files (can only load a .html file, no local CSS/JS, can't give a blank address bar for me to search in), so it doesn't solve the issue either. This is very important to me, and others have been asking for this feature back for years. It feels awful to have the working solution stop working, in the browser that's supposed to be open and customizable.

ChromeUtils.importESModule("resource:///modules/AboutNewTab.sys.mjs").AboutNewTab.newTabURL = "file:///home/razv/StartPage/index.html";