Can I design the Popup as I desire?

FireBox comes with the new WordPress editor, Gutenberg, which helps you create a beautiful page easier and faster, using a list of blocks that you can move around and customise as you desire.

There are a ton of 3rd-party plugins that provide further blocks and thus giving you endless possibilities on what you can create.

Apart from the Gutenberg editor, you may also use the classic editor and populate it via shortcodes and HTML.