Let’s do a full clean re-install; Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86)

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Do Not remove the profiles folder.

Look for, and rename any Mozilla or Firefox program folders by adding .old to them. Do not rename the Mozilla Thunderbird folder if there is one.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. If all goes well, remove the OLD folders when you are done. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

bvdiane said

A couple of weeks ago I found your fix for this, telling me to go to New Tab Override (browser.newtab.url replacement) https://addons.mozilla.org/en-US/firefox/addon/new-tab-override/. I do that, and the undefined goes away for an hour or a day, then comes right back.

Can you track that time frame more specifically? For example, let's say that the change works until you completely exit out of Firefox, and the problem returns the next time you start Firefox. Those kinds of issues are covered in this article: How to fix preferences that won't save.

It's certainly also possible that you have "battlin' add-ons" and the problem is caused by another one of your enabled extensions. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious? If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox before the change takes effect. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

bvdiane said

Also, when I open a new tab, my frequently used pages don't show up (i.e. Facebook, Google News, etc.), it's just a blank page.

The New Tab Override extension normally replaces the standard Firefox new tab page with another page of your choice. If you want the standard Firefox new tab page, you don't need the New Tab Override extension. Does that make sense?

But as long as you're using it, the internal address of the built-in new tab page is about:newtab so you could enter that in NTO's settings and see whether that gets you the right page. To adjust NTO's settings, you'll go back to the Add-ons page, Extensions list, find New Tab Override and click its Options button. You may need to scroll down past some general information to find the settings section of the page.

You can check the current new tab page with this code in the Browser Console.

• set devtools.chrome.enabled to true on the about:config page to enabled the command line
• https://developer.mozilla.org/en/Tools/Browser_Console

Cu.import("resource:///modules/NewTabURL.jsm");
NewTabURL.get();