There's an obvious reason: preventing bugs. If I were to make a bank transfer, and the process hanged in the final step because of an incompatible browser, I'd

There's an obvious reason: preventing bugs. If I were to make a bank transfer, and the process hanged in the final step because of an incompatible browser, I'd be furious, even if I had been warned about the risk using FF. In the worse case, I could be wondering if I need to repeat the operation with Chrome, or if the failure occurred after the transfer and repeating would duplicate... If they don't test their website with FF, there's a higher risk of bugs. A FF user might not even realize that they're hindered by bugs: the site could be much slower, or some blocks would not refresh, or some transactions fail silently. If it doesn't clearly look broken, and just behaves badly, the user will probably blame the website instead of switching to a Chromium-based browser.