WordPress Membership Plugin
I did a bit more digging. I hadn’t realized the Plugin added to the user table (just went live yesterday).
When registering through Firefox, I am able to successfully register and then login.
However, when I create a user through the Simple Membership Plugin (via Add New), that user cannot be logged in through Firefox, but can log in via Chrome or IE or Safari.
The only difference was that when I checked the WordPress Users, every one of the registrations that came in from the browser registration was shown with a site role of “Participant”. The one I set up internally had the site role of the membership level I set up in Simple WordPress.