A feature that has changed recently in Firefox is content blocking. There may be a shield to the left of the URL of the bank. Click on it, and you can turn off blocking for the site you are on. Then try the site again, and if it still does not work clear your cache and try the site again. If that did not fix the problem, go to webcompat.com and file a report about the site not working with Firefox.
You can try the same thing with the stock site. Proper testing would be done with a new Firefox profile. The more effort you put into it, the more likely the problem is to eventually be fixed.