I’ve just been playing round with add-ons and found that Privacy Badger seems to be the culprit. If I disable it in Firefox and refresh the page I have no problem accessing our website. Surprisingly if I disable Privacy Badger for this site within Privacy Badger, there is still a problem.
It seems to be okay at the moment, hopefully it’s fixed for good. If everything is okay again tomorrow I’ll mark this solved.
Thanks for the help guys.