M's online banking refused to work yesterday, having been fine for years.
With Firefox, it got to the "Confirm payment" bit, then dropped back to the account summary and didn't make the payment.
I tried with Chrome, and it worked fine.
So I went back and looked more carefully. One of my Firefox plugins is HTTPS-Everywhere. It seems the bank now has a non-HTTPS tracker running alongside the HTTPS main site, and that offends the plugin. I think I agree with the plugin about that.