From my understanding after looking into this, you should only need to point the new tab URL to your local file using the file:// protocol via:
user_pref(“browser.newtab.url”, “file:///home/username/path/to/page.html”);
Yet, any CSS, JavaScript, and images must be on your local disk as well and referenced using the absolute paths or the relative paths from the HTML file’s location. If you still run into issues following these steps, it’s probably the Content Security Policy blocking the scripts or external resources of your local HTML file. Before setting it as the permanent new tab, try testing the file manually first with:
file:///home/youruser/path/to/page.html
Also, be sure all the styles and scripts load correctly, then apply the config method resulting in a successful test.
The preference goes in the “user.js” file inside the profile directory, should be found in about:support by searching for the Profile Directory/Folder. It’s probably something like:
~/.librewolf/< random-string >.default/
You need to create/edit the user.js file, here’s the command:
nano ~/.librewolf/<your-profile>/user.js
Then add the line:
user_pref(“browser.newtab.url”, “file:///home/username/path/to/page.html”);
After this, save and restart the browser. There’s a chance this just won’t work as browser.newtab.url was removed ~ FireFox 40. It was replaced with an extension-based API, so ya, this might not work in modern LibreWolf… But it’s worth a try. Otherwise, an extension like New Tab Override should let you set a file:// path for new tabs. It hooks into the browser.newtab API that replaced these older preferences, but I know you haven’t any much luck either way.