Do you still get the about:newtab page?

If you do not want the about:newtab page to open in a new tab then set the browser.newtab.url pref to about:blank on the about:config page.

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

thanks Cor-el - I've done as you suggested first with about:home and secondly about:blank but neither made any difference - I still get the Top sites when I open a new tab.

Start Firefox in Safe Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

If you do not keep changes after a restart then see:

• http://kb.mozillazine.org/Preferences_not_saved
• https://support.mozilla.org/kb/Preferences+are+not+saved