I believe Firefox and Chrome do not allow you to set a local html file as a new tab page. You can set it as a home page but then you need to manually navigate to it after creating a new tab.
I use Chrome. I build a static HTML file and have set a New Tab page policy[1] in my paid Google Workspace[2] admin console
to use this static HTML file via a file:// URL.
Now I had to try. I can make Firefox to open my Startpage whenever I open a new window, but not for each new tab. Chrome only as the first window after I start the browser. (MacOS)
[1] https://support.google.com/chrome/a/answer/2657289#zippy=%2C...
[2] https://workspace.google.com/