--- a/testing/mozbase/rust/mozrunner/src/runner.rs
+++ b/testing/mozbase/rust/mozrunner/src/runner.rs
@@ -351,31 +351,35 @@ fn find_binary(name: &str) -> Option<Pat
     })
 }
 
 #[cfg(target_os = "linux")]
 pub mod platform {
     use super::find_binary;
     use std::path::PathBuf;
 
+    /// Searches the system path for `firefox`.
     pub fn firefox_default_path() -> Option<PathBuf> {
         find_binary("firefox")
     }
 
     pub fn arg_prefix_char(c: char) -> bool {
         c == '-'
     }
 }
 
 #[cfg(target_os = "macos")]
 pub mod platform {
     use super::find_binary;
     use std::env;
     use std::path::PathBuf;
 
+    /// Searches the system path for `firefox-bin`, then looks for
+    /// `Applications/Firefox.app/Contents/MacOS/firefox-bin` under both `/`
+    /// (system root) and the user home directory.
     pub fn firefox_default_path() -> Option<PathBuf> {
         if let Some(path) = find_binary("firefox-bin") {
             return Some(path);
         }
         let home = env::home_dir();
         for &(prefix_home, trial_path) in [
             (
                 false,
@@ -404,16 +408,19 @@ pub mod platform {
 #[cfg(target_os = "windows")]
 pub mod platform {
     use super::find_binary;
     use std::io::Error;
     use std::path::PathBuf;
     use winreg::RegKey;
     use winreg::enums::*;
 
+    /// Searches the Windows registry, then the system path for `firefox.exe`.
+    ///
+    /// It _does not_ currently check the `HKEY_CURRENT_USER` tree.
     pub fn firefox_default_path() -> Option<PathBuf> {
         let opt_path = firefox_registry_path().unwrap_or(None);
         if let Some(path) = opt_path {
             if path.exists() {
                 return Some(path);
             }
         };
         find_binary("firefox.exe")
@@ -454,16 +461,18 @@ pub mod platform {
         c == '/' || c == '-'
     }
 }
 
 #[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
 pub mod platform {
     use std::path::PathBuf;
 
+    /// Returns `None` for all other operating systems than Linux, macOS, and
+    /// Windows.
     pub fn firefox_default_path() -> Option<PathBuf> {
         None
     }
 
     pub fn arg_prefix_char(c: char) -> bool {
         c == '-'
     }
 }