Your credit union is setting a cookie in response to you entering the sign-in code. Firefox is likely configured in some way to delete cookies when you quit the browser. Check for

Settings -> Privacy & Security -> Cookies and Site Data -> Delete cookies and site data when Firefox is closed (Un-check the box.)

and

Settings -> Privacy & Security -> History -> Clear history when Firefox closes (If enabled, click the Settings button, then un-check Cookies.)

If this isn't exactly the Firefox setup you want, you can refine this process through Settings -> Privacy & Security -> Cookies and Site Data -> Manage Exceptions. I expect there are also Addons that will give you more granular cookie control.

Hope that helps.

You can use these steps to make a website recognize and remember you.

• create a cookie allow exception with the proper protocol (https:// or http://) to make a website remember you

You can check that you aren't clearing important cookies.

• using "Delete cookies and site data when Firefox is closed" to clear cookies keeps cookies with an allow exception
  in 102+ version toggling this setting makes changes to the "Clear history when Firefox closes" settings and those settings prevail
• using "Clear history when Firefox closes" in Firefox 102+ honors exceptions and keeps cookies with an allow exception, previous versions removed all cookies

Make sure to keep the "Site settings".

• clearing "Site settings" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Exceptions"
• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings