Revert "Hide option for browser.download.useToolkitUI in Firefox 26+"
Doesn't work, if browser.download.useToolkitUI is missing

This reverts commit e7eb50889bb443913691682962b6b361a4db4eeb.
Hide option for browser.download.useToolkitUI in Firefox 26+
(now correctly...)
Mark compatible with Firefox 29.0a1