With Firefox 41 (and higher), it's no longer possible to customize the page shown in a new tab via changing the preference browser.newtab.url in about.config. Because of the fact that hijackers abused the preference in the past, Mozilla decided to remove it (see bug 1118285). Fortunately, by removing it, Mozilla also introduced a new API to bring this functionality back to life as an add-on. This add-on allows the user to choose a certain page in a new tab.

New Tab Override (browser.newtab.url replacement) https://addons.mozilla.org/en-US/firefox/addon/new-tab-override/

This is also available using Classic Theme Restorer (Customize Australis) {web link} Restore squared tabs, appmenu, add-ons bar, small button view and more on Australis UI (Windows/MacOSX/Linux + Fx 29+).

In case the above doesn't solve the problem :
This could be caused by corrupted cookies or cookies that are blocked
Clear the cache and cookies only from websites that cause problems.

Clear the Cache:

3-bar menu => Options => Advanced => Network => Cached Web Content : "Clear Now"

Remove Cookies from sites causing problems:

3-bar menu => Options => Privacy => Cookies : "Show Cookies"

Thank you both. I think I'm in good shape now.

Glad to help. Safe Surfing.