You're ignoring the fact that "Firefox Home" itself allows you to customize what it contains. You can remove the recommendations, shortcuts, recent pages, recent bookmarks etc. individually (or strip it all the way down to a search bar).

> You're ignoring the fact that "Firefox Home" itself allows you to customize what it contains.

Correct. I am ignoring that fact because I don't want new tabs to display "Firefox Home" and I am not interested in customizing what it contains.

I want new tabs to display an HTML file that I made myself. Firefox previously allowed doing so.

The replies that suggest using extensions to re-enable the disabled functionality probably have not actually tried doing so. What those extensions do is load the "Firefox Home" content, then (as if on page load) redirect to whatever you set as your custom new tab. Which is just wretched. I could have typed the URL faster than that.

And on the subject of pushing core browser functionality into extensions: Why are things like Pocket, Firefox Sync, Hello, and the like integrated / bundled with Firefox but the ability to set a custom URL for new tabs requires users to install / trust a third party extension?

See also: The deplorable state of "Classic Theme Restorer", another case of "Users who miss the disabled functionality can just install an extension."

That explains the ton of extensions to manage that page. That does seem like a pretty dark pattern move, I recall it having three options, the last being a custom URL.

There’s extensions to change the new tab page

It's absurd that it needs an extension. Another example of how Firefox no longer acts as a user agent.