diff -b -r -c geckodriver-0.24.0/src/marionette.rs geckodriver-0.24.0-horn/src/marionette.rs
*** geckodriver-0.24.0/src/marionette.rs	Mon Jan 28 23:26:07 2019
--- geckodriver-0.24.0-horn/src/marionette.rs	Wed Mar  6 16:31:23 2019
***************
*** 54,59 ****
--- 54,63 ----
  use crate::capabilities::{FirefoxCapabilities, FirefoxOptions};
  use crate::logging;
  use crate::prefs;
+ use regex::Regex;
+ use regex::NoExpand;
+ use std::fs;
+ use std::fs::File;
  
  #[derive(Debug, PartialEq, Deserialize)]
  pub struct MarionetteHandshake {
***************
*** 171,176 ****
--- 175,190 ----
              runner.arg("-jsdebugger");
          }
          if let Some(args) = options.args.as_ref() {
+ 	    let mut found = false;
+ 	    for i in 0..args.len() {
+ 		if found {
+ 		    try!(self.update_port_in_profile(port, &args[i]));
+ 		    break;
+ 		}
+ 		if args[i] == "-profile" {
+ 		    found = true;
+ 		}
+ 	    }
              runner.args(args);
          }
  
***************
*** 185,190 ****
--- 199,217 ----
          Ok(())
      }
  
+     fn update_port_in_profile(&mut self, port: u16, profile_dir: &String) -> WebDriverResult<()>
+     {
+ 	let mut filename: String = profile_dir.to_owned();
+ 	filename.push_str("/prefs.js");
+ 	let content = fs::read_to_string(&filename).expect("Unable to read prefs.js");
+ 	let re = Regex::new(r#"marionette.port", [0-9]*"#).unwrap();
+ 	let tmp = format!(r#"marionette.port", {}"#, port);
+ 	let result = re.replace(&content, NoExpand(&tmp));
+ 	let mut file = File::create(&filename).expect("Unable to open prefs.js");
+ 	file.write_all(result.as_bytes()).expect("Unable to write to prefs.js");
+ 	Ok(())
+     }
+ 
      pub fn set_prefs(
          &self,
          port: u16,