As well as having heard it from many prominent security experts, from personal experience now I've come to expect there's a trade-off between security and the human need for convenience. Despite there being a slew of faster and more reliable browsers than Firefox I choose to use it for the extensions which make my browsing more secure and private, while accepting that such plugins may break some sites.
I can now add my university email account to the mix: after an interface upgrade whenever I attempt to login to it with the Permit Cookies extensions enabled I can't get past the above screen even if I add the network to my whitelist. It is the first time a site has still refused to play nicely with this plugin even after whitelisting, very weird.
Looks like I'll be setting up my beloved Camino browser to take care of university whatnot. Now there's a slick, fast and lightweight browser!
In the meantime I'll contact the university IT help desk. Given this is the science and engineering campus here I'm sure I'm not the only Firefox and Permit Cookies users here having this problem, at least I hope I'm not!