Currently document.activeElement may return the input element inside type="file". Content JS gets then an exception when it tries to access the element.
Comment on attachment 620830 [details] [diff] [review] patch https://tbpl.mozilla.org/?tree=Try&rev=52113d2992a5
Bah, the patch revealed another bug.
Created attachment 621005 [details] [diff] [review] +fix to test_bug430351.html https://tbpl.mozilla.org/?tree=Try&rev=527d379c12aa
Neil, the patch doesn't change the focusing behavior, but it is just that test_bug430351.html tests currently the_native_anon_button_inside_file_input != <input type="file"> With the patch that becomes <input type="file"> != <input type="file">