|Sorry, by AV I meant "Antivirus". The website looked quite normal to me.|
Nope Firefox has no Firewall - it's just a browser. Rightly or wrongly I got the impression that you had another software firewall (part of your Antivirus maybe) and as an aside felt that it was a bad idea because software can clash and cause issues.
Right now I'm thin on ideas but if it is still the same with your Antivirus and firewall(s) turned off then about the only other thing that comes to mind is the router firewall. Obviously you wouldn't want to run with its firewall disabled but if temporarily doing so helps then we could try and find out why.
To get to your router controls you put a number in your browser address bar. It varies with routers but, for example, mine is 192.168.1.1 and yours is likely to be similar (192.168.2.1 maybe). It should be in your route documentation somewhere or you could Google its make and model number to find it. This allows you to change various parameters and one of these will be firewall on or off. There might be a password to get into the controls but if you haven't changed it from default you will probably have to type in Admin or maybe just hit the Enter key.
I have a feeling there is something we are both missing, so I'll keep thinking - maybe in the meantime someone else will pop by with some fresh ideas.
Always pop back and let us know the outcome - thanks