SKY log in access denied on Firefox

Been with firefox a long time. After recent update I am unable to login to SKY a/c via toolbar / Google search usinf Firefox. Log in works with Microsoft Edge. E-m client accesses e-mails. (Previous Thunderbird as e-mail client collapsed ). All other log in / site access on toolbar is OK, only SKY. SKY have checked their side completely - nothing amiss on the line / router etc. No VPN in use either. All fully checked out, SKY confident that it is not their end, works with Edge but no way on Firefox. Hence seems to be a bug local to Firefox and one unique log in.
Dunno if anyone can help / suggest.
I am not techno clever in any way !

¬°Hola @Piepowder!

https://validator.w3.org/nu/?doc=https%3A%2F%2Fskyid.sky.com%2Fsignin shows there are some warnings and errors on the page.

Please bring these to the attention of Sky.

¬°Gracias!