It sounds like you are using about:newtabs as your home page.

Type about:preferences#general<enter> in the address bar.

There are two settings;

• When Firefox Starts

When the browser is started, what do you want to display. Many of us choose Show My Home Page.

• Home Page

When a new page is opened, what do you want displayed.

1. Use Current Page. Use what ever page(s) are open at that time.
2. Use Bookmark
3. Restore To Default. about:home shows a Mozilla home page with tools.

You can use any of these that you wish;

about:home (Firefox default home page), about:newtab (shows the sites most visited), about:blank (a blank page),

or you can enter any web page or about: page you want.

Your System Detail list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

You can check if you have a user.js file in the Firefox profile folder that sets the browser.sessionstore.resume_session_once pref to true.

• http://kb.mozillazine.org/browser.sessionstore.resume_session_once

You can check its content with a plain text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

This post addresses a new tab you open in an existing window using, for example, the "+" button on the tab bar or the keyboard shortcut Ctrl+t. If you meant a completely new window, this reply is not applicable to your problem.

On the built-in new tab page, you can use the "gear" or "wheel" icon in the upper right to switch off the display of thumbnails/tiles. See this article for more information: Customize your New Tab page to hide or display content. If your new tab page doesn't resemble the one in the article, it may be generated by an add-on.

If you want to use a completely different page on a new tab, you need an add-on to make the change. For full details, see this thread: https://support.mozilla.org/questions/1089474