Generally speaking, you won't use Eclipse directly for a task like this. If you are writing automated tests using a tool like Appium or Selenium, you will likely want to open the app under test in a mainstream browser such as Chrome or Firefox. Firefox seems to be the more popular choice for development/testing, so I'd recommend that.
If you are testing a mobile web app, I'd still recommend opening it up in a desktop browser using Firefox since the tools are pretty good already. You can run tests in whatever environment makes sense, but you should consider developing tests using a mainstream browser like Firefox or Chrome.
To identify elements you would like to locate using xpath queries or otherwise, use the Firebug extension as noted in other answers.