Member asked to log in on every page

I have a member who would be asked to log in on every single page, even after logging in.

She said she is running a Mac with Yosemite, fully updated Firefox browser, and no ad blocking software. Her membership level has access to the pages she is attempting to view, and I have verified it is working for other users on the same level.

Any ideas as to why this is happening and how to prevent it in the future?

Thanks!