A couple of things, its a new server, and its also now SSL. I have a forced permanent redirect from HTTP to HTTPS so it may be that Firefox isn't picking this up correctly on your machine. Its working fine on mine. I'd suggest typing the url in as :-
https://cbfsim.co.uk and clicking on the forum button, then update your bookmarks.
WRT the earlier comment about the site on phones etc, almost al new websites (phpBB included) are now what's known as a responsive website, basically, the site adapts automatically to the screen size of the client device. Again, its been extensively tested on a myriad of devices (including iPhones, iPads, Kindles, Android phones, Windows Phones, and with a variety of browsers. The lout dynamically changes, but the conent is all there, but you may find the layout is subtly different.
https://en.wikipedia.org/wiki/Responsive_web_design
As you can see from the attached screenshot from my phone, the layout is different to the desktop site, but its all there.