I have not been able to log in to Citibank to pay my credit card on my linux box, using Firefox. An open google search led to a blog that suggested installing the firefox plugin that disables flash. This worked great for me. The plugin link is: https://addons.mozilla.org/en-US/firefox/addon/433
As an FYI, you can enable the flash options manually as you need them, and you can whitelist any sites you want flash to work with automatically by selecting the menu: tools>>extensions. Select the plugin in the pop-up box that comes up, and click the preferences button.
I still cannot log into the CapitalOne site - it keeps timing out. Does anyone have a solution for that website? Maybe the problem is just temporary and on their end?