There is a bug in the sendKeys functionality for sending inputs to a textarea while testing on Firefox 53. For reference, please see the related GitHub issue for the fix that was implemented with geckodriver 0.16.0.
In order to successfully test on Firefox 53, you will need to use Selenium 3.4.0 or above (which will use geckodriver 0.16.0 or higher). This is also the default for Firefox 53 tests if the you don't explicitly declare a seleniumVersiondesired capability.
If you choose use an older version of Selenium (any version before 3.4.0) in tests, then an older version of GeckoDriver will be used and the sendKeys error will occur.