Firefox runs just fine on your Windows 8.1 computer. Can you give the exact error message your bank is giving?

Also, you should update to Windows 10 ASAP, 8.1 is not as secure or supported, and you should update. I believe you can use your windows 8.1 product key to get a free windows 10 update

Firefox Releases since 53.0 and later including including the current 59.0.3 still requires Windows 7, 8 (8.1), 10. https://www.mozilla.org/firefox/59.0/system-requirements/

You useragent in you More System Details on right of post shows as Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:59.0) Gecko/20100101 Firefox/59.0 in which NT 6.3 is Windows 8.1.

Firefox itself is a web browser and not a OS or such so what do you mean by "Firefox has updated to use windows 10"

I wonder if you may be confused by the "Fix for compatibility with Windows 10 April 2018 update" mentioned in https://www.mozilla.org/firefox/59.0.3/releasenotes/

yes I can give you the exact message----that message was"SECURE CONNECTION FAILED"