As a workaround, you can install a UserAgent Switcher extension for firefox, which will let you change firefox's useragent on the fly. I set it back to Netscape 4.8, and can see the login scree on HSBC's website.
This sort of workaround is only a good thing while trying to get the problem solved so you can do what you need to do. Long term, when the site operators check their stats for their site they obliviously say "see, everyone's using browser X", which is bad.
Yeah. No shit. However, long term fixes don't help the OP do his online banking right now, and it's the right now problems that stop people using linux long term. Don't get me wrong, I think HSBC should sort their shit out and fix their website. Incidentally, changing banks isn't the easiest solution. What if you have to move your two mortgages, chequing account, savings account, *and* update all your bill payments and salary deposits? Rebooting into windows sounds much easier to me.