please check in firefox > options > advanced > network > connection - settings... if firefox is set up to directly connect to the internet ('no proxy').

Firefox can't load websites but other browsers can

this is an issue that might also be caused by some sort of firewall or internet monitoring/parental control system on your computer.

Thanks for the quick response. I should have mentioned that no proxy is being used. I had discounted firewall and parental controls and even add-ons because the https request without the www prefix was working.

The domain name is long (18 characters) and includes a hyphen, I'm wondering if www. making 22 characters (26 if include .com) is longer than some internal firefox limit. Does anyone else have experience of long (or non-alphabetic characters) domain names on https requests?