Previously, if you didn't set the desired capabilities for platform and version for automated website tests, Sauce would default to Linux for the platform capability, and either Chrome 48 or Firefox 45 for version, depending on what you selected for browserName. However, there are more recent versions of both of these browsers available for other platforms, so we have updated the default platform and browser version for situations in which you don't set these capabilities. Now, if you don't set platform and version, your tests will default to Windows 7 for platform, and either Chrome 59 or Firefox 54 for version.
Because these more recent browser versions use different drivers from the previous versions, your tests may behave differently, or break, if your tests are set to use the default platform and browser for Sauce Labs. Specifically, Chrome 59 uses ChromeDriver 2.29, while Chrome 48 uses ChromeDriver 2.21. Firefox 45 uses the legacy FirefoxDriver which runs Selenium 2.53.1, while Firefox 54 uses geckodriver 0.16, which runs Selenium 3.4.0.
To make sure your tests continue to run as expected, you should either specify a platform and version in your tests, or update your test functionality to work with the newer versions of ChromeDriver, geckodriver, and Selenium.