B.O.A.C....No script worked with mine also...I have uninstalled it now as it was more trouble than it was worth....Anyway, the problems you are having might suggest it's your copy of FF that is playing up, Perhaps you should uninstall it again and do a complete new install...See if that works.
Just to reiterate, it worked fine with the Santander site. I even signed up for the more secure login system with it. As a matter of interest are you logging in from the new Santander site or the old Abbey site?
Also, as a matter of interest, Abbey site always told me I was using an incompatible browser, however I used to just carry on and it worked fine....