I get a sign-in error, "This browser is not supported or 3rd party cookies and data may be disabled."

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.

Chrome

1. Navigate to chrome://settings/content/cookies
2. 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).

Firefox

1. Navigate to about:preferences#privacy
2. Under "History", choose "Firefox will remember history"