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

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

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

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

I used Safe Mode & checked the add-ons, and it looks like a new FF interface. The article listed says to "click the button in the upper-right corner of the page" and to use the "pin icon" to set up the New Tab page, but I don't have those icons on my pages. I get a new tab (see attached), but it says only "You've opened a new tab".

What is the value of the browser.newtab.url pref on the about:config page?

• http://kb.mozillazine.org/about:config

You can try to reset some preferences to the default with the SearchReset extension:

• https://addons.mozilla.org/firefox/addon/searchreset/

Note that the SearchReset extension only runs once and then uninstalls automatically, so it won't show on the "Firefox > Add-ons" page (about:addons).

If you are not able to keep changes permanently (e.g. they revert after restarting Firefox) then see:

• http://kb.mozillazine.org/Preferences_not_saved

You can check for problems with preferences and try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default values.

• http://kb.mozillazine.org/Resetting_preferences
• http://kb.mozillazine.org/Preferences_not_saved

• https://support.mozilla.org/kb/Preferences+are+not+saved
• https://support.mozilla.org/kb/Resetting+preferences

Thanks! The "searchreset" tool worked. It changed the contents of "browser.newtab.url" from something quite long & messy to "about:newtab", and now it works like it's supposed to. Next I have to figure out how to pin my favorite pages to the new tab page . . .

You can change the order of the boxes on the about:newtab page and drag them to a different position to pin them to that position or you can remove them by clicking the close X of an item to block them.
You can drag a bookmark or history item and drop them in a slot on the about:newtab page to pin them.

Changes manually made on the about:newtab page are stored in the chromeappsstore.sqlite file in the Firefox Profile Folder under the pinnedLinks key and blockedLinks key.

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

It6's a good concept except I find that clicking the pin on each box simply rearranges all the pages randomly.