Sorry, but the New Tab Override extension has been updated to use the more restricted WebExtensions API for Firefox 57 compatibility and can no longer show a local page (a page whose address starts with file://) -- or apparently a chrome:// page -- in place of the built-in new tab page.

As a "two-click" workaround you can pin the local/chrome page in the first tile position on the built-in new tab page. See: Customize your Firefox New Tab page.

Or if you display the Bookmarks Toolbar you can bookmark your local page on the bar and Ctrl+click or middle-click the bookmark to launch it in a new tab.

Hopefully one of those will tide you over until someone comes up with a better way.

Thanks, it's a shame it's not really possible to have this functionality in the newer Firefox versions. Hopefully some day they'll add it back in

This still works for me with a mozilla.cfg auto configuration file to set the new tab page.

//
var newTabURL = "chrome://browser/content/bookmarks/bookmarksPanel.xul";
Cu.import("resource:///modules/NewTabURL.jsm");
NewTabURL.override(newTabURL);

You can use the mozilla.cfg file in the Firefox program folder to set or lock preferences and run privileged JavaScript code.

The mozilla.cfg file needs to be in the main Firefox program folder.

This requires a local-settings.js file in the "defaults/pref" folder where the channel-prefs.js file is located that specifies to use mozilla.cfg.

pref("general.config.filename", "mozilla.cfg");
pref("general.config.obscure_value", 0);

The mozilla.cfg file and possibly local-settings.js need to start with a comment line (//).

See Configuration:

• https://developer.mozilla.org/en-US/Firefox/Enterprise_deployment

See also:

• http://kb.mozillazine.org/Locking_preferences
• http://mike.kaply.com/2012/03/16/customizing-firefox-autoconfig-files/

Thanks, I'll give that a try