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

Author: Delta456

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 +680,12 @@ class Builder {
         if (this.firefoxService_) {
           service = this.firefoxService_.build()
         }
+        // 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?.args.includes('--connect-existing')) {
+          service?.args.append('--websocket-port')
+          service?.args.append(`${portprober.findFreePort()}`)
+        }
         return createDriver(firefox.Driver, capabilities, service)
       }