[Launchpad] launching firefox from the dev server causes it to crash

[Kenji Okamoto]

STR:

1. quit any debugee firefox open browsers
2. run yarn start in debugger.html
3. go to localhost:8000 and click 'Launch Firefox'

Within a few minutes the dev server crashes with this error

WebDriverError: connection refused
    at Object.throwDecodedError (/debugger.html/node_modules/selenium-webdriver/lib/error.js:489:11)
    at parseHttpResponse (/debugger.html/node_modules/selenium-webdriver/lib/http.js:513:13)
    at doSend.then.response (/debugger.html/node_modules/selenium-webdriver/lib/http.js:432:13)
    at process._tickCallback (internal/process/next_tick.js:68:7)
From: Task: WebDriver.createSession()
    at Function.createSession (/debugger.html/node_modules/selenium-webdriver/lib/webdriver.js:777:24)
    at Function.createSession (/debugger.html/node_modules/selenium-webdriver/firefox/index.js:667:55)
    at createDriver (/debugger.html/node_modules/selenium-webdriver/index.js:167:33)
    at Builder.build (/debugger.html/node_modules/selenium-webdriver/index.js:642:16)
    at Object.start (/debugger.html/node_modules/devtools-launchpad/bin/firefox-driver.js:91:6)
    at isFirefoxRunning.then (/debugger.html/node_modules/devtools-launchpad/src/server/launch.js:26:23)
    at process._tickCallback (internal/process/next_tick.js:68:7)
error Command failed with exit code 1.