I have FF 9.01 as well and have just signed in to my own Yahoo.co.uk email account without problem.
I doubt the .uk version is any different, so I am unsure where the problem lies.
Try another browser first to see if you can get access through that. If so, then it's looking like FF is at fault and would look at your FF Add-ons to see if there is anything that might be causing this.
Just recently updated to Firefox 9.01. Everything seems fine, EXCEPT when I try to access Yahoo mail. Every time I go to Yahoo Mail, Firefox freezes and must be restarted. Am I missing an extension or a plug in?
Anyone else with this problem?