There's a couple of ways you could set your own startpage as the new tab page, but my preferred method is to use web-ext, a command-line tool for building, running and testing web extensions for Firefox and Chromium-based browsers, maintained by the Mozilla team.

Prerequisites

You'll need to install web-ext before you proceed to the next section. I would suggest searching for it and installing it from your distribution's package repositories.

Manifest

Your extension will need to define a manifest which describes in JSON what it's called, what it does, what permissions it needs, etc.

# manifest.json
{
    "manifest_version": 2,
    "name": "Startpage",
    "version": "1.0.0",
    "description": "New Tab Startpage",
    "chrome_url_overrides": {
      "newtab": "index.html"
    }
}

Take this manifest for example which, besides the usual metadata, states that it would like to override chrome_url_overrides.newtab by setting it to index.html (relative to manifest.json).

Your browser will gladly comply because the WebExtensions API (which is mostly browser-agnostic) allows for such modifications.

Tinkering

When we're building or tinkering with an extension, we're constantly making changes, testing things out and repeating the same process until we're finally satisfied with the result.

There's a command which can temporarily load the extension in the browser so that we can efficiently and immediately see the results of the changes we make to the add-on. In the same directory as manifest.json, run the following command:

web-ext run

You can target a specific browser using the --target flag, e.g. if you're using Chromium or a Chromium-based browser, provide chromium as an argument and add the --chromium-binary flag, specifying the path of the Chromium(-based) binary, e.g. $(whereis chromium).

Building

Once you're done testing out your add-on, you'll want to package the add-on as a .zip file. To do that, visit the same directory as manifest.json and run the following command:

web-ext build

Signing

The last and most crucial part of this guide is the signing process. Firefox for example requires add-ons to be signed by Mozilla before we can install or self-distribute them, keep in mind that any changes made to the source code of the extension will only be reflected once the add-on is re-built and re-signed.

In the same directory as manifest.json, run the following command:

web-ext sign --api-key=$AMO_JWT_ISSUER --api-secret=$AMO_JWT_SECRET

For more information on how to obtain your --api-key and --api-secret, read the official documentation.

If all goes well during the signing process, you will discover that a new directory named web-ext-artifacts has appeared and contains a .xpi file.

Installing

Firefox users can install the .xpi file like so:

1. Enter “about:addons” in the search bar
2. Drag and drop the .xpi file onto the page or locate and click on the gear button and select "Install Add-on from File", then manually pick the .xpi file
3. Accept the prompt to add the extension

That's it, you're all set!