The Comment security level dropdown does not align correctly under firefox when it is opened over an image.

As seen in bad_scroll.mp4.

Steps:

1. http://jira.atlassian.com/browse/GHS-2705
2. Scroll so the image is just below the stalker.
3. Click on comment in the stalker.
4. Click on the comment security level.

Actual:

The page scrolls and the drop down is displayed in the middle of the page.

Expected:

The drop down just works.