I've been testing a new site with Selenium. Part of this work was on my Mac and in addition to running the tests in Firefox, I wanted to use Safari too. Whenever I launched the test case the first driver window opened, but the content window didn't ever appear. No errors, no fuss, it just didn't happen.
After walking away from the problem for a while, the penny finally dropped........Safari blocks pop-ups by default! Turning that off instantly solved all my problems.