Most likely, this means your browser is blocking third-party cookies. Tab for a Cause uses a third party for sign-in, so you'll have to enable them.
- Navigate to chrome://settings/content/cookies
- Turn off "Block third-party cookies"
Or, if you want to keep blocking most third-party cookies: in the "Allow" section, add tab-for-a-cause.firebaseapp.com (the site we use for sign-in).
- Navigate to about:preferences#privacy
- Under "History", choose "Firefox will remember history"