--- a/devtools/client/framework/ToolboxProcess.jsm
+++ b/devtools/client/framework/ToolboxProcess.jsm
@@ -208,24 +208,29 @@ BrowserToolboxProcess.prototype = {
 
   /**
    * Originally, the profile was placed in `ProfLD` instead of `ProfD`.  On some systems,
    * such as macOS, `ProfLD` is in the user's Caches directory, which is not an
    * appropriate place to store supposedly persistent profile data.
    */
   _migrateProfileDir() {
     let oldDebuggingProfileDir = Services.dirsvc.get("ProfLD", Ci.nsIFile);
+    let newDebuggingProfileDir = Services.dirsvc.get("ProfD", Ci.nsIFile);
+    if (oldDebuggingProfileDir.path == newDebuggingProfileDir.path) {
+      // It's possible for these locations to be the same, such as running from
+      // a custom profile directory specified via CLI.
+      return;
+    }
     oldDebuggingProfileDir.append(CHROME_DEBUGGER_PROFILE_NAME);
     if (!oldDebuggingProfileDir.exists()) {
       return;
     }
     dumpn(`Old debugging profile exists: ${oldDebuggingProfileDir.path}`);
     try {
       // Remove the directory from the target location, if it exists
-      let newDebuggingProfileDir = Services.dirsvc.get("ProfD", Ci.nsIFile);
       newDebuggingProfileDir.append(CHROME_DEBUGGER_PROFILE_NAME);
       if (newDebuggingProfileDir.exists()) {
         dumpn(`Removing folder at destination: ${newDebuggingProfileDir.path}`);
         newDebuggingProfileDir.remove(true);
       }
       // Move profile from old to new location
       let newDebuggingProfileParent = Services.dirsvc.get("ProfD", Ci.nsIFile);
       oldDebuggingProfileDir.moveTo(newDebuggingProfileParent, null);