Type about:config<enter> in the address bar. If a warning screen comes up, press the Be Careful button. At the top of the screen is a search bar. Type browser.newtab.url This tells Firefox what to show when a new tab is opened. If you want, right click and select Modify. You can change the setting to;

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.

The same instructions are used for the new window setting, listed as browser.startup.homepage.

super.molfar. Thank you for your reply. I typed 'about:config<enter>' into my address bar and nothing happened, except a message which said 'That address doesn't look quite right'. What am I doing wrong??? Michael Phillips

Maybe this helps? https://www.wikihow.com/Change-your-Start-Page-on-Mozilla-Firefox