@f-meunier seems more like a browser caching problem maybe? If you at first had accessed it via the lan IP?
There should be nothing different be it the browser accesses it via the wan IP or the lan IP.
That being said - access from the wan/internet to the gui is normally a bad idea.. Unless you can lock it down to specific source IP or network.. Say from your work IP/Network or something.
Bad idea to expose the gui to the public internet.. If you need/want to access pfsense web gui while your remote - via a vpn connection is far better method.
I do access some of works pfsense boxes via the wan, locked down to my home IP as the only source. And I have not noticed any issues using firefox. They are not yet on 2.5.2 because of lack of access to facility, and risk in updating when nobody could be on site to correct if something went wrong.. But I do have test 2.5.2 setup here, that I could access via its wan IP vs its lan IP.. If you could give some specific example of what your seeing so I could try and duplicate it. But again - there would be no difference accessing it via the wan or lan IP. If wan rules allowed the access.