Anyway, even if I did have a compatible user agent switcher plug in, very few of them include the latest released version of the browser in the pre-programmed list which again makes it hard to convince the banks that you are running the latest stable software.
So here are the instructions on how to set the user agent string yourself on Firefox
Goto the browser address bar:
Right mouse click to get the context menu and choose New->String from the menu.
As the preference name.
Then enter this as the value
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:126.96.36.199) Gecko/20070309 Firefox/188.8.131.52
For the average user running Windows XP, this should be fine to get past the pedantic banking sites who don't have a robust way of checking the browser version.