Using the web interface setting up CUPS on CURRENT, I end up very often with a blank frame/screen within Firefox, where the information changes or newly made setups should occur. I can avoid this by being faster than the refesh timout (the small time amount the refresh waits before trying to refresh/redirect) and hitting the link presented after a change has been made. In cases I'm faster, the refreshed screen is correct and as it is expected. If I wait for the  redirection, I end up in a situation were I can only start from the initial CUPS page, neither reload, nor back button work.

[...]
cups-2.0.3_1
Name           : cups
Version        : 2.0.3_1
Installed on   : Tue Jul 21 07:00:26 CEST 2015
Origin         : print/cups
Architecture   : freebsd:11:*
Prefix         : /usr/local
Categories     : print

[...]
firefox-41.0.2,1
Name           : firefox
Version        : 41.0.2,1
Installed on   : Mon Oct 19 06:37:24 CEST 2015
Origin         : www/firefox
Architecture   : freebsd:11:x86:64
Prefix         : /usr/local

and FreeBSD is
FreeBSD 11.0-CURRENT #9 r289991: Mon Oct 26 05:52:29 CET 2015 amd64

I checked for know issues and I found a bug fix reported at cups.org targetting an redirect issue, but I'm not sure this is the problem I face in my setups.