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

DO NOT MERGE

This is not yet ready to merge as needs testing on a demo server, but please feel free to review and test locally.

https://bugzilla.mozilla.org/show_bug.cgi?id=903192

The bug is a little confusing as it references IE8 specifically and the popup blocker's 'high' setting. Through testing however, this actually appears to effect IE6/7/8 on the regular 'medium' setting as well.

See this comment for an explanation as to what causes the behavior:
https://bugzilla.mozilla.org/show_bug.cgi?id=903192#c30 

This fix is for the clicking the download button on the `firefox/new` page. Currently the file download prompt will get blocked on these IE browsers by a security blocker. This fix circumvents the blocker on all three browsers when the setting is 'medium'. It also works on IE8 when the setting is 'high'.

IE9 and above, as well and non-IE browsers should remain unaffected by this change.