Use the Firefox reset to fix this - Refresh Firefox - reset add-ons and settings

A Reset is swiftest, but if you want to try something more surgical, you might be able to reset the "new tab" back to its default behavior using the about:config preferences editor.

(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 filter box, type or paste newtab and pause while the list is filtered

(3) Double-click the browser.newtab.url preference and enter the desired value:

(i) Page thumbnails (default)

browser.newtab.url = about:newtab

(ii) Blank tab

browser.newtab.url = about:blank

(iii) Built-in Firefox home page

browser.newtab.url = about:home

(iv) Any other page

browser.newtab.url = full URL to the page

Press Ctrl+t to open a new tab and verify that it worked. Fixed?

See also this article about the New Tab page (about:newtab):

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

• https://support.mozilla.org/kb/How+to+set+the+home+page

The mystart toolbar is known to cause a lot of issues. Using the reset will also get rid of it.

Thanks guys! I did try jscher2000's method but couldn't get it right, so I used reset instead.