The Node configuration file that I used.

ConsoleLogs.txt

java -jar selenium-server-standalone-3.3.0.jar -role node -hub http://localhost:4444/grid/register -nodeConfig NodeConfig.json -debug true
08:57:28.622 INFO - Selenium build info: version: '3.3.0', revision: 'b526bd5'
08:57:28.623 INFO - Launching a Selenium Grid node
08:57:30.432 DEBUG - CookieSpec selected: default
08:57:30.443 DEBUG - Auth cache not set in the context
08:57:30.444 DEBUG - Connection request: [route: {}->http://localhost:4444][total kept alive: 0; route allocated: 0 of 2000; total allocated: 0 of 2000]
08:57:30.549 DEBUG - Connection leased: [id: 0][route: {}->http://localhost:4444][total kept alive: 0; route allocated: 1 of 2000; total allocated: 1 of 2000]
08:57:30.550 DEBUG - Opening connection {}->http://localhost:4444
08:57:30.601 DEBUG - Connecting to localhost/127.0.0.1:4444
08:57:30.613 DEBUG - Connect to localhost/127.0.0.1:4444 timed out. Connection will be retried using another IP address
08:57:30.613 DEBUG - Connecting to localhost/0:0:0:0:0:0:0:1:4444
08:57:30.614 DEBUG - http-outgoing-0: Shutdown connection
08:57:30.614 DEBUG - Connection discarded
08:57:30.615 DEBUG - Connection released: [id: 0][route: {}->http://localhost:4444][total kept alive: 0; route allocated: 0 of 2000; total allocated: 0 of 2000]
08:57:30.615 WARN - error getting the parameters from the hub. The node may end up with wrong timeouts.Connect to localhost:4444 [localhost/127.0.0.1, localhost/0:0:0:0:0:0:0:1] failed: Connection refused
2017-03-11 08:57:30.805:INFO::main: Logging initialized @5810ms to org.seleniumhq.jetty9.util.log.StdErrLog
08:57:31.025 INFO - Driver provider org.openqa.selenium.ie.InternetExplorerDriver registration is skipped:
 registration capabilities Capabilities [{ensureCleanSession=true, browserName=internet explorer, version=, platform=WINDOWS}] does not match the current platform MAC
08:57:31.025 INFO - Driver provider org.openqa.selenium.edge.EdgeDriver registration is skipped:
 registration capabilities Capabilities [{browserName=MicrosoftEdge, version=, platform=WINDOWS}] does not match the current platform MAC
08:57:31.025 INFO - Driver class not found: com.opera.core.systems.OperaDriver
08:57:31.025 INFO - Driver provider com.opera.core.systems.OperaDriver registration is skipped:
Unable to create new instances on this machine.
08:57:31.025 INFO - Driver class not found: com.opera.core.systems.OperaDriver
08:57:31.025 INFO - Driver provider com.opera.core.systems.OperaDriver is not registered
2017-03-11 08:57:31.481:INFO:osjs.Server:main: jetty-9.2.20.v20161216
2017-03-11 08:57:31.722:INFO:osjsh.ContextHandler:main: Started o.s.j.s.ServletContextHandler@4b0b0854{/,null,AVAILABLE}
2017-03-11 08:57:31.914:INFO:osjs.AbstractConnector:main: Started ServerConnector@72f926e6{HTTP/1.1,[http/1.1]}{0.0.0.0:5555}
2017-03-11 08:57:31.915:INFO:osjs.Server:main: Started @6920ms
08:57:31.915 INFO - Selenium Grid node is up and ready to register to the hub
08:57:31.941 DEBUG - Using the json request : {"class":"org.openqa.grid.common.RegistrationRequest","name":null,"description":null,"configuration":{"hubHost":"localhost","hubPort":4444,"id":"http://192.168.1.3:5555","capabilities":[{"seleniumProtocol":"WebDriver","firefox_binary":"/opt/firefox-50.1.0/firefox","browserName":"firefox","maxInstances":3,"version":50,"platform":"LINUX"},{"seleniumProtocol":"WebDriver","firefox_binary":"/opt/firefox-51.0.1/firefox","browserName":"firefox","maxInstances":3,"version":51,"platform":"LINUX"}],"downPollingLimit":2,"hub":"http://localhost:4444/grid/register","nodePolling":5000,"nodeStatusCheckTimeout":5000,"proxy":"org.openqa.grid.selenium.proxy.DefaultRemoteProxy","register":true,"registerCycle":5000,"unregisterIfStillDownAfter":60000,"cleanUpCycle":2000,"custom":{},"host":"192.168.1.3","maxSession":5,"servlets":[],"withoutServlets":[],"browserTimeout":0,"debug":false,"jettyMaxThreads":null,"log":null,"port":5555,"role":"node","timeout":1800}}
08:57:31.942 INFO - Starting auto registration thread. Will try to register every 5000 ms.

NodeConfig.json

{
  "registerCycle": 5000,
  "hub": "http://xx.xx.xx.xx:4444",
  "cleanUpCycle": 2000,
  "register": true,
  "capabilities": [
    {
      "maxInstances": 3,
      "seleniumProtocol": "WebDriver",
      "firefox_binary": "/opt/firefox-50.1.0/firefox",
      "platform": "LINUX",
      "browserName": "firefox",
      "version": 50
    },
    {
      "maxInstances": 3,
      "seleniumProtocol": "WebDriver",
      "firefox_binary": "/opt/firefox-51.0.1/firefox",
      "platform": "LINUX",
      "browserName": "firefox",
      "version": 51
    }
  ],
  "nodeStatusCheckTimeout": 5000,
  "role": "node",
  "maxSession": 5,
  "port": 5555
}

However ... I just ran it without the -debug parameter that it's complaining about, using your node.json file and ... it works. I see just the two versions of Firefox listed on the console that are specified in the node.json file, rather than the default options.

Now I need to figure out why your version of my file works, but mine doesn't ... is it bothered about item ordering in the file perhaps?