diff --git a/testing/geckodriver/src/main.rs b/testing/geckodriver/src/main.rs
index 5dd1201f6c40..3fbf1c56f304 100644
--- a/testing/geckodriver/src/main.rs
+++ b/testing/geckodriver/src/main.rs
@@ -19,7 +19,7 @@ use std::net::{SocketAddr, IpAddr};
 use std::path::Path;
 use std::str::FromStr;
 
-use argparse::{ArgumentParser, IncrBy, StoreTrue, Store};
+use argparse::{ArgumentParser, IncrBy, StoreTrue, Store, StoreOption};
 use webdriver::server::start;
 
 use marionette::{MarionetteHandler, BrowserLauncher, LogLevel, MarionetteSettings, extension_routes};
@@ -46,7 +46,7 @@ struct Options {
     binary: String,
     webdriver_host: String,
     webdriver_port: u16,
-    marionette_port: u16,
+    marionette_port: Option<u16>,
     connect_existing: bool,
     e10s: bool,
     log_level: String,
@@ -59,7 +59,7 @@ fn parse_args() -> Options {
         binary: "".to_owned(),
         webdriver_host: "127.0.0.1".to_owned(),
         webdriver_port: 4444u16,
-        marionette_port: 2828u16,
+        marionette_port: None,
         connect_existing: false,
         e10s: false,
         log_level: "".to_owned(),
@@ -81,7 +81,7 @@ fn parse_args() -> Options {
             .add_option(&["--webdriver-port"], Store,
                         "Port to run webdriver on");
         parser.refer(&mut opts.marionette_port)
-            .add_option(&["--marionette-port"], Store,
+            .add_option(&["--marionette-port"], StoreOption,
                         "Port to run marionette on");
         parser.refer(&mut opts.connect_existing)
             .add_option(&["--connect-existing"], StoreTrue,
@@ -205,6 +205,8 @@ mod tests {
     use mozprofile::preferences::Pref;
     use std::io::Read;
 
+    const MARIONETTE_DEFAULT_PORT: u16 = 2828;
+
     #[test]
     fn test_profile() {
         let mut profile_data = Vec::with_capacity(1024);
@@ -227,7 +229,7 @@ mod tests {
         };
 
         let settings = MarionetteSettings {
-            port: 2828,
+            port: None,
             launcher: BrowserLauncher::None,
             e10s: false,
             log_level: None,
@@ -235,7 +237,7 @@ mod tests {
         let handler = MarionetteHandler::new(settings);
 
         let mut gecko_profile = handler.load_profile(&capabilities).unwrap().unwrap();
-        handler.set_prefs(&mut gecko_profile, true).unwrap();
+        handler.set_prefs(MARIONETTE_DEFAULT_PORT, &mut gecko_profile, true).unwrap();
 
         let prefs = gecko_profile.user_prefs().unwrap();
 
diff --git a/testing/geckodriver/src/marionette.rs b/testing/geckodriver/src/marionette.rs
index f5d849bfd5c4..91d53f8d1afe 100644
--- a/testing/geckodriver/src/marionette.rs
+++ b/testing/geckodriver/src/marionette.rs
@@ -16,7 +16,7 @@ use std::io::ErrorKind;
 use std::io::Result as IoResult;
 use std::io::prelude::*;
 use std::io;
-use std::net::TcpStream;
+use std::net::{TcpListener, TcpStream};
 use std::path::{Path, PathBuf};
 use std::sync::Mutex;
 use std::thread::sleep;
@@ -277,7 +277,7 @@ impl FromStr for LogLevel {
 }
 
 pub struct MarionetteSettings {
-    pub port: u16,
+    pub port: Option<u16>,
     pub launcher: BrowserLauncher,
 
     /// Enable or disable Electrolysis, or multi-processing, in Gecko.
@@ -293,7 +293,7 @@ pub struct MarionetteHandler {
     connection: Mutex<Option<MarionetteConnection>>,
     launcher: BrowserLauncher,
     browser: Option<FirefoxRunner>,
-    port: u16,
+    port: Option<u16>,
     e10s: bool,
     log_level: Option<LogLevel>,
 }
@@ -315,7 +315,11 @@ impl MarionetteHandler {
         debug!("create_connection");
         let profile = try!(self.load_profile(capabilities));
         let args = try!(self.load_browser_args(capabilities));
-        match self.start_browser(profile, args) {
+        let port = match self.port {
+            Some(x) => x,
+            None => try!(get_free_port())
+        };
+        match self.start_browser(port, profile, args) {
             Err(e) => {
                 return Err(WebDriverError::new(ErrorStatus::UnknownError,
                                                e.description().to_owned()));
@@ -323,7 +327,7 @@ impl MarionetteHandler {
             Ok(_) => {}
         }
         debug!("Creating connection");
-        let mut connection = MarionetteConnection::new(self.port, session_id.clone());
+        let mut connection = MarionetteConnection::new(port, session_id.clone());
         debug!("Starting marionette connection");
         try!(connection.connect());
         debug!("Marionette connection started");
@@ -331,7 +335,7 @@ impl MarionetteHandler {
         Ok(())
     }
 
-    fn start_browser(&mut self, profile: Option<Profile>, args: Option<Vec<String>>) -> Result<(), RunnerError> {
+    fn start_browser(&mut self, port: u16, profile: Option<Profile>, args: Option<Vec<String>>) -> Result<(), RunnerError> {
         let custom_profile = profile.is_some();
 
         match self.launcher {
@@ -340,7 +344,7 @@ impl MarionetteHandler {
                 if let Some(cmd_args) = args {
                     runner.args().extend(cmd_args);
                 };
-                try!(self.set_prefs(&mut runner.profile, custom_profile));
+                try!(self.set_prefs(port, &mut runner.profile, custom_profile));
                 try!(runner.start());
 
                 self.browser = Some(runner);
@@ -353,10 +357,10 @@ impl MarionetteHandler {
         Ok(())
     }
 
-    pub fn set_prefs(&self, profile: &mut Profile, custom_profile: bool)
+    pub fn set_prefs(&self, port: u16, profile: &mut Profile, custom_profile: bool)
                  -> Result<(), RunnerError> {
         let prefs = try!(profile.user_prefs());
-        prefs.insert("marionette.defaultPrefs.port", Pref::new(self.port as i64));
+        prefs.insert("marionette.defaultPrefs.port", Pref::new(port as i64));
 
         prefs.insert_slice(&FIREFOX_REQUIRED_PREFERENCES[..]);
         if !custom_profile {
@@ -1085,6 +1089,12 @@ impl ToJson for MarionetteError {
     }
 }
 
+fn get_free_port() -> IoResult<u16> {
+    TcpListener::bind(&("localhost", 0))
+        .and_then(|stream| stream.local_addr())
+        .map(|x| x.port())
+}
+
 pub struct MarionetteConnection {
     port: u16,
     stream: Option<TcpStream>,
@@ -1105,6 +1115,7 @@ impl MarionetteConnection {
         let poll_interval = 100;  // ms
         let poll_attempts = timeout / poll_interval;
         let mut poll_attempt = 0;
+        info!("Connecting to Marionette on localhost:{}", self.port);
         loop {
             match TcpStream::connect(&("localhost", self.port)) {
                 Ok(stream) => {