[FF] Opening preview using keyboard will cause popup block

[Marek Lewandowski]

since: 4.0 (didn't check earlier) until master

1. open any sample with CKEditor (i.e. samples/replacebyclass.html)
2. focus editor
3. press alt+f10 to focus toolbar
4. navigate to preview button
5. press button using space

Expected result:
Preview should appear

Current result:
Preview is blocked by popup block

additional info:

1. preview is not blocked when we do this using mouse
2. tested with FF 26.0 at Win7