Is it possible to detect Firefox users in pure CSS?

Nowadays you can use the @supports CSS rule:

@supports not( -moz-appearance:none ){
  /* Add non-firefox CSS code here */
}

Not that I know of, but you can try this:

@-moz-document url-prefix() {
 
    #my-id { font-size: 100%; }
 
}

This website has more options as well

You can place this in your CSS file or between your <style type='text/css'> tags in your HTML. Works Fine!