Is there a way to update your Firefox without replacing the look or your preferences? Like putting a new engine in a car but the car is still the same car?

Hi, all fine but no info about your Firefox version, extensions, video card drivers were sent nor supplied so have no idea what you would like to do as no info about what version you are running.

Currently, there is no way to run Firefox 57 or higher without the new user interface design. But: since before last November, users have been posting creative solutions to modify the appearance of the user interface. Whether that involves curving tabs, showing text on buttons, moving bars up and down, or other changes, many things are possible with custom style rules. However, you do need to get specific about what is most important to you, or it's impossible to give specific recommendations.

To give you a general idea: https://www.userchrome.org/what-is-userchrome-css.html