Reproducible using Firefox 42 beta 2 under Mac OS X 10.9.5. Not reproducible using Dev Edition 43.0a2 and Nightly 44.0a1 2015-09-30. Steps to reproduce: 1. Open a private window 2. Enter full screen 3. From about:privatebrowsing page select "See how this works" Expected results: The privacy tour starts Actual results: The browser window is put in background. The browser console error is: The resource at "https://trackertest.org/" was blocked because tracking protection is enabled.[Learn More]

Fwiw, I can't reproduce this with Firefox 42 beta on Mac OSX 10.10.5. The browser console message is also expected, since the page intentionally loads a resource from https://trackertest.org/ to trigger the Tracking Protection feature. This is a domain we set up specifically for the tour, so it's not a real tracker.

I tried on a different machine with 10.10.5 and I could reproduce there too. It seems that it only happens in a second tab, after already starting the tour in another tab. This way I could see it on Nightly and Dev Edition too. Would a screencast help?

> Would a screencast help? Yes please

I hope it's more clear now: https://drive.google.com/file/d/0B7jBNZBM3mz-QUxGYmRVMkVsb00/view?usp=sharing

That's a strange one, thanks for reporting.