This happens when Tab for a Cause is unable to store a cookie in your browser and there are a few quick fixes to try:
1. Check if you have either an extension or browser setting that is disabling all cookies. If so, you might be able to make an exception for tab.gladly.io
2. A recent Firefox update means that the new tab page is treating cookies from tab.gladly.io as 3rd party cookies which are often blocked by default. To fix, add tab.gladly.io to the cookie whitelist in Firefox.
Firefox Cookie whitelist instructions:
Open FF settings then navigate to Privacy and Security. Under Cookies and Site data select "manage permissions." Add "tab.gladly.io" and select "allow"