Follow these steps to set your custom Frontpage (or any other URL) as the default "New-Tab"-Page in your Firefox Browser:
1. Install the "New Tab Override" Add-on
There are several different free Add-ons which do the same thing. We like this free one and will show you how to use this one. If you choose a different one the steps may be a bit different.
First of all you need to install the extension by opening Firefox and navigating to this page on the Firefox Add-On directory. Once you clicked on "Add to Firefox", the Add-on will be added to your Firefox browser.
2. Configure the Add-on
The first time you open up a new Browser Tab and you'll see a "New Tab - Settings" Configuration page.
If Firefox asks you if this is the expected page, click on "Keep these changes".
Now, all you need to do is enter the URL of your Frontpage, e.g. "https://myfront.page/example" in the input field labeled "URL" and press the Enter-key to save it and you are ready to go!
Now open up a new tab again and you should see your custom Frontpage! If Firefox asks you again if this is the expected page, click on "Keep these changes".
Whenever you open up a new tab in Firefox from now on, you will be greeted with your custom Frontpage! Awesome!
Additional infos
Please note that we did not develop this Add-on. We aren't liable for anything the Add-on does.
If this particular Add-on does not work with your version of Firefox, check out the Firefox Add-on directory to find alternative Add-ons which do the same thing.
If you wish to reverse those changes and want to see your default Firefox "New-Tab"-Page again, open the Firefox Menu, click on "Add-ons" and press "Remove" next to the "New Tab Override" Add-on (visible after clicking the icon with the 3 dots next to the Add-on name).