Sliding Windows

I hate to post anything as what I am describing may be a feature! - but one that is driving me nuts! I had two windows open on my iMac (Firefox and Pages) - reading a Pages Forum and trying out some of the suggestions. I put Pages in the Dock and later - when I clicked on Pages to continue - FireFox flew to the Dock and Pages flew out. So I clicked on FireFox and they traded places again. I could get the two to sit side-by-side. I restarted, cleaned up permissions in single user mode - and it was fine again. However, this has repeated several more times and is not predictable. - Is it FireFox?

So what silly thing am I not thinking of?