[bug 903192] IE popup blocker prevents users from downloading Firefox

This PR been tested on demo1 server by @retornam and it all works ok, but it needs code review and also some more testing from other folks.

I'm able to reproduce the issue where the file download prompt gets blocked on 'medium' setting, but Raymond has been unable to do so on his setup. More testers on this issue would be good.

To see if you experience the issue, go to the following page on IE 6/7/8 with popup blocker set to 'medium' (the default setting) and click the download button.

http://www.mozilla.org/en-US/firefox/new/

If a security prompt blocks the file download (which then requires another click to allow it), then you can reproduce the issue. This is what this PR aims to circumvent. If you can't reproduce, please comment also.

IE9 and above, and non-IE browsers should remain unaffected by this change.

Note: the original bug references the 'high' setting for the popup blocker.  I don't think there's any way to circumvent the security dialog on this setting - but during my testing I noticed it was still happening on 'medium' which is where this PR came from.