[js] add --websocket-port flag when not connecting to existing connection on firefox

javascript/selenium-webdriver/index.js

@@ -44,6 +44,7 @@ const BrowsingContext = require('./bidi/browsingContext')
 const BrowsingContextInspector = require('./bidi/browsingContextInspector')
 const ScriptManager = require('./bidi/scriptManager')
 const NetworkInspector = require('./bidi/networkInspector')
+const portprober = require('./net/portprober')
 const version = require('./package.json').version
 
 const Browser = capabilities.Browser
@@ -679,6 +680,15 @@ class Builder {
         if (this.firefoxService_) {
           service = this.firefoxService_.build()
         }
+
+        // Find a free port before starting Firefox
+        const port = portprober.findFreePort();
+
+        // Allocate a free port for the websocket only when not connecting to an existing instance
+        // This avoids conflicts when multiple Firefox instances have started
+        if (!service?.options_.args.includes('--connect-existing')) {
+          service?.addArguments('--websocket-port', `${port}`)
+        }
         return createDriver(firefox.Driver, capabilities, service)
       }