When using Spaces (Mission Control) in OS X, switching to a space with Firefox at the front, will always move the second to last app that was in the front in front of Firefox.
Here's how to reproduce:
1. Bring any other app to the front (focus).
2. Bring Firefox to the front.
3. Switch to another space.
4. Switch back to the space with Firefox. The other app will be in front of Firefox.
Firefox is the only app I've noticed so far that does this.