After importing JSON of successful recording in Selenium builder, the test case execution fails on first step: GET.
It just opens Firefox browser and doesn't instrument it.
It happen with Mozilla FireFox version 45 and above.
It is due the fact that FireFox version 45 and above needs Selenium builder updated libraries of version 2.53 and above.
1. Replace the current Selenium JARs you have under <DevTest_Home>/lib/shared/ folder with JARs from version 2.53.
2. They can be downloaded from - http://www.seleniumhq.org/download/
3. Go to section Selenium Client & WebDriver Language Bindings.
Download from here:
Language Client Version Release Date
Java 2.53.0 2016-03-15