Over the last few days I’ve had a problem accessing my online banking (in this case Alliance and Leicester) from Firefox. Internet Explorer was fine.
After running Firefox in safe mode (run firefox -safe-mode) I realised the issue was with one of my add-ons. With a hunch I launched Firefox normally and disabled Firebug. Sure enough, I could now access my banking.
I don’t know why this change has occurred – possibly A&L have upgraded their site security. Anyway I can live with FireBug disabled until I need to use it.