Make a user style in Stylish, Stylus or other extension for your browser. The original CSS gets modified like this:

 

@-moz-document domain("your local domain here") {
    #header {
        background-size: 1200px 90px;
    }
}

 

Specifying the size of the banner image overrides the default "100%" width and stops it from resizing as you resize the browser window. Put in the size of your own custom image - default (full) height seems to be 90px.

 

 

 

 

Edited July 28, 20241 yr by Espressomatic

Examples on three different systems:

 

 

 

Edited July 29, 20241 yr by Espressomatic