I really don't believe the return-on-investment for a major bank to support Firefox is negative. Quick Google napkin math says a customer is worth $500/year, and a decent programmer is easily available for $50/hour. You only need to stop one (1) customer from leaving for every day of programming this costs. This thread is at +89 and has 32 comments, so it's probably worth at least a programmer-month.

If you can't make your website Firefox compliant in a month, you have some very deep design issues. Firefox compliance is so trivial that most websites achieve it without even trying, because Firefox follows basically the same standards Chrome does.