F posted
I resist upgrading my software for as long as possible because IME the newer versions always turn out to be worse than the earlier ones, in some respects at least.
Nevertheless I have taken your advice and upgraded to 28.0, and hey presto, the BT e-billing site now works.
However, as expected, Firefox 28 has drawbacks that its aged predecessor didn't have. In particular, you can't easily turn off Javascript, which is something I have to do a lot in my work (I can't be bothered to explain why). You have to do it via about:config instead of Tools.Options.Content.