Write test pages for certain FF24 features

There are a couple of new Firefox features that may behave in unacceptable ways on some platforms. In particular, the web notifications api (https://developer.mozilla.org/en-US/docs/WebAPI/Using_Web_Notifications) could potentially introduce proxy bypass similar to what we saw with drag and drop url sniffing by the OS Desktop.

Similarly, if https://developer.mozilla.org/en-US/docs/Web/API/ClipboardEvent.clipboardData is able to randomly inspect the clipboard, this could be very privacy invasive. We should ensure that the clipboard APIs either ask the user first, or otherwise only interact with clipboard data originating from that same page.

There were also some changes to the Download Manager in terms of how it executes helper apps, and to the external app launcher's threading behavior. We should verify that our external app blocker still asks the user for confirmation in these cases: https://bugzilla.mozilla.org/show_bug.cgi?id=858234 https://bugzilla.mozilla.org/show_bug.cgi?id=789932

Finally, support for querying and inspecting font variants was introduced. Do our font limit counters still apply in that case? Should they? https://bugzilla.mozilla.org/show_bug.cgi?id=549861