Firefox on Debian stable is so old that websites yell at you to upgrade to a newer browser. And last time I tried installing Debian testing (or was it debian unstable?), the installer shat itself trying to make the bootloader. After I got it to boot, apt refused to work because of a missing symlink to busybox. Why on earth do they even need busybox if the base install already comes with full gnu coreutils? I remember Debian as the distro that Just Wroks(TM), when did it all go so wrong? Is anyone else here having similar issues, or am I doing something wrong?
How do you know this? Of course there are lots of reasons for why they'd want to enforce minimum browser versions. But security might very well be one of them. Especially if you're a bank you probably feel bad about sending session tokens to a browser that potentially has known security vulnerabilities.
And sure, the user agent isn't a sure way to tell whether a browser is outdated, but in 95% of cases it's good enough, and people that know enough to understand the block shouldn't apply to them can bypass it easily anyway.