I'm assuming you can see some of the Firefox user interface, such as the orange Firefox button on the frame of the window, or the classic menu bar? And you see your tabs?

To turn the Navigation Toolbar on and off, try right-clicking either:

• a blank area of the tab bar
• the new tab ("+") button on the tab bar

to pop up the toolbar list so you can turn individual bars on and off.

Does that work?

Regarding the page that loads in the new tab, Firefox normally will display 9 thumbnails for sites you frequently visit, or pages you've added yourself. If you see or want to see something different, there's a hidden setting.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste newtab and pause while the list is filtered

(3) Double-click the browser.newtab.url preference and enter your preferred page:

• Page thumbnails (default) => about:newtab
• Blank tab => about:blank
• Built-in Firefox home page => about:home
• Any other page => full URL to the page

You can press Ctrl+t to open a new tab and verify that it worked.

• If Firefox ignores the setting, one of your extensions may be overriding it. You can review, disable, and/or remove extensions on the add-ons page:
  
  orange Firefox button (or Tools menu) > Add-ons > in the left column click Extensions

• If the change works but at the next startup has changed on its own, you might have a user.js file in your personal Firefox settings folder (your Firefox profile folder). This article describes how to track down and remove the file: How to fix preferences that won't save.

See also:

• Tools > Options > General > Startup: Home page
• https://support.mozilla.org/kb/How+to+set+the+home+page

This article about the New Tab page (about:newtab):

• https://support.mozilla.org/kb/new-tab-page-show-hide-and-customize-top-sites