Let me try to get this in.
Using a second (instance/profile of the same brand) browser, I asked for the pfsense page and was given the login page. However, this page is using a different theme.
Comparing the cookies between the two browsers, I find that the first browser has three cookies named PHPSESSID with different 'host' values. Deleting the two obviously wrong same-named cookies got me past the login page.
On the first browser, I logged in and was given the dashboard page of the theme I always used.
On the second browser, a different theme is shown, yet the General Setup, Theme choice is 'pfsense_ng' in both browsers.
Edit: Changing the selection of the theme does not change the theme displayed.
Edit2: Correction - the theme changes on the first browser when a selection is made, but not on the second when a selection is made.
So, I think the browser was sending too many cookies or the wrong cookie.
Suggest that pfSense use a cookie with a name unique unto itself, instead of the common/default PHPSESSID.