In Firefox, you can choose a local html file as your home page, but not as a new tab page. This is allegedly because of some security concerns. Using extentions allows for a limited workaround where the page needs to be re-imported each time it is edited.

The surprising part to me is that the same applies to the forks too.

If opening a local file for the home page not a security concern, why should it be for the new tab page. I understand that giving local files access to extensions could lead to issues, however, it should not need an extension to use a local file as a new tab page.

Note: I maintain my bookmarks in a local html file, which I make into my home page, new tab page, across browsers, and then sync this file across devices using Syncthing.