You don't need anything fancy. Start with any sample "New Tab" extension for chrome and firefox found on github, strip it back to being basically blank then add whatever you want. Enjoy! I really like my custom new tab extension and have tinkered with it for years

Since I don't have mine in a public repo, a cursory search shows a good example here. The manifest.json bit here is the only thing needed iirc.

https://github.com/kaissaroj/chrome-newtab/blob/master/manif...